Confirm what? That is where they camped and what they did. The authorities certainly believed it was them.
Beck saw it as a psychic experience.
From the link in your OP:
The events leading up to the ape episode were filled with the psychic element. Since a young man I had always been clairvoyant. When just a boy I was in the pasture playing with my beanshooter. I had bought it with some long earned coins. It had a twisted wire handle. I lost it, and as I was crying, a kindly woman came up to me and put her arms around me. I felt warm all over. "Little boy," she said, "don't cry. Go home, you will find your beanshooter there."
I went home and found it, and as far as I knew then it was the same one. But years later I found the one I lost. It was weather beaten and the rubber was rotten.
I would be sleeping on the hard benches of the Adventist Church my folks used to attend, and I would have my head in a lady's lap, only when I mentioned it to my folks, they said there was no one else there and took it to be a boy's musings.
As I grew older, I saw visions and eventually I was holding spiritual meetings. After 1924 I spent many years in healing work.
Our time spent in Mt. St. Helens was a series of psychic experiences. The method we found our mine was psychic. I am mentioning these facts to help build a background of understanding in a case which has been wholly explained in a material sense. As I will explain the mysterious realities of the Abominable Snowmen, so must I show some of my inner experiences, for in my mind the two have always been closely connected together; and as I will show, these beings bear a direct association with the psychic realm.
In 1922 we found the location of our mine. A spiritual being, a large Indian dressed in buckskin, appeared to us and talked to us. He was the picture of stateliness itself. He never told us his name, but we always called him the Great Spirit. He replied once, "The Great Spirit is above me. We are all of the Great Spirit, if we listen when the Great Spirit talks."
There was another spiritual being which appeared to us — more in the role of a comforting friend, and we learned her name. One of our party suggested later that we name our mine after her; and so the mining claim we later filed bore her last name. The big Indian being told us there would be a white arrow go before us. Another man, who was not present during the attack in 1924, could see the arrow easily and clearly at all times. And I could see it nearly as well.
And on and on it went. With arrows apparently flying through the air by itself to lead them to where they needed to blast for their mine shaft, and so on and so forth. Apparently one spirit was angry they were talking with another spirit and did not give them the gold, and after years of trying to find gold, they gave up. The story became more and more fantastic as it went on.
Beck described bigfoot as manifestations.
The picture I am trying to paint for you is a picture of manifestation. In this book I have showed also some of our high manifestation, as we had many of a high nature the six years we spent in the Mt. St. Helen's area. Of course, the lower or grosser manifestations were in the appearing of these hairy creatures called Abominable Snowmen — also called Hairy Apes, Bigfoot and Sasquatch, according to the region they are seen in. I believe they have always existed, although our encounter with them in 1924 was the first major report of anyone coming in contact with them. Other writings have documented the cases very well. I just like to show the reason for their appearances.
It seems the spirits were the good or greater manifestations and bigfoot the lesser ones.
So to him, these weren't even real creatures, but ones from the spirit world. He even says that they were not real creatures.
Another very striking experience which shows that they cannot be natural beings with natural bodies: It was before we made our cabin, and we were staying in a tent then. The tent was below a little cone shaped mountain called Pumy Butte. A little creek flowed nearby, and there was a moist-sand bar about an acre in area. We would go there and wash our cooking utensils and bring our drinking water back. Early one morning Hank came back to the tent. He was rather excited. He led us to the moist-sand bar, and took us almost to the center. There in the center of the sand bar were two huge tracks about four inches deep. There was not another track on that sand bar!
There we were standing in the middle of the sand bar, and not one of us could conceive any earthly thing taking steps 160 feet long. "No human being could have made these tracks," Hank said, "and there's only one way they could be made, something dropped from the sky and went back up."
There was no third step. This is certainly another indication of what I'm saying about manifestation. I have heard it said that many ages ago the Rocky Mountain and Cascade Mountain Ranges were a center of a great civilization. I do think the mountain areas are extra sensitive to spiritual vibration, usually of a higher order, but sometimes lower. We ourselves being extra sensitive to spiritual vibration, probably had come into contact with the manifestation of these being easier than, perhaps, the average person would have.
On the subject of the Snowmen I am not going to say Masters told me this or that. It would perhaps look impressive, but I want to keep the record straight. Masters have appeared in our home and talked with us, but not once do I recall asking them anything on the subject. Since that day in 1924, I have went on and progressed and have learned much, and now I can look back and put the puzzle together from the reservoir of knowledge I have learned.
In the true sense everything in the material world is a manifestation. Ever since the time the first essence of consciousness formed from the Great Void we cannot describe, different planes or dimensions of being were created or manifested. Occasionally we of this dimension of space can be conscious of other beings of a different vibration and consciousness.
The The Abominable Snowmen are from a lower plane.When the condition and vibration is at a certain frequency, they can easily, for a time, appear in a very solid body. They are not animal spirits, but also lack the intelligence of a human consciousness When reading of evolution we have read many times conjecture about the missing link between man and the Anthropoid Ape. The Snowmen are a missing link in consciousness, neither animal nor human. They are very close to out dimension, and yet are a part of one lower. Could they be the missing link man has been so long searching for?
So bigfoot wasn't real to Beck.
It was a lesser spirit from another dimension.
The kids from the YMCA camp sounds much more plausible than a guy and apparently with his friends who appeared to be on a magical mushroom trip.