But the translation is spurious, the word 'only' isn't there anywhere
bhaktyā — by pure devotional service; mām — Me; abhijānāti — one can know; yāvān — as much as; yaḥ ca asmi — as I am; tattvataḥ — in truth; tataḥ — thereafter; mām — Me; tattvataḥ — in truth; jñātvā — knowing; viśate — he enters; tat-anantaram — thereafter
the word tattvatah is repeated twice
perhaps you would have a case if you could produce a verse that explicitly states how krishna can be known through brahman (since there are numeorus statements that declare brahman can be known through krishna)
While Krishna certainly prefers devotion over all methods, he also mentions knowledge (as well as karma yoga, dhyana yoga, etc...):
and these descriptions of different yoga conclude in the devotional aspect
eg
BG 6.47: And of all yogīs, the one with great faith who always abides in Me, thinks of Me within himself, and renders transcendental loving service to Me — he is the most intimately united with Me in yoga and is the highest of all. That is My opinion.
BG 5.29: A person in full consciousness of Me, knowing Me to be the ultimate beneficiary of all sacrifices and austerities, the Supreme Lord of all planets and demigods, and the benefactor and well-wisher of all living entities, attains peace from the pangs of material miseries.
"By becoming fixed in this knowledge, one can attain to the transcendental nature like My own. Thus established, one is not born at the time of creation or disturbed at the time of dissolution" (BG 14.2)
this verse is an intro to the subject of the three gunas, but even the next verse establishes that krishna is the source of brahman
BG 14.3: The total material substance, called Brahman, is the source of birth, and it is that Brahman that I impregnate, making possible the births of all living beings, O son of Bharata.
also confirmed in Mundaka Upanisad (3.1.3)
“One who sees that golden-colored Personality of Godhead, the Supreme Lord, the supreme actor, who is the source of the Supreme Brahman, is liberated.”
(the sanskrit is brahma-yonim - source of brahman)
"All these devotees are undoubtedly magnanimous souls,
meaning as previously established
BG 7.16: O best among the Bhāratas, four kinds of pious men begin to render devotional service unto Me — the distressed, the desirer of wealth, the inquisitive, and he who is searching for knowledge of the Absolute.
all 4 of these devotees are great souls
but he who is situated in knowledge of Me I consider to be just like My own self.
meaning one who fulfills the criteria of the previous verse
BG 7.17: Of these, the one who is in full knowledge and who is always engaged in pure devotional service is the best. For I am very dear to him, and he is dear to Me.
is considered to be situated in full knowledge
Being engaged in My transcendental service, he is sure to attain Me, the highest and most perfect goal" (BG 7.18)
confirms the path of bhakti
As for Supreme Lord, he is unknowable, like Brahman, but the origin of Brahm, the origin of the unborn...
the reason that brahman is considered difficult for one who is embodied
BG 12.5: For those whose minds are attached to the unmanifested, impersonal feature of the Supreme, advancement is very troublesome. To make progress in that discipline is always difficult for those who are embodied.
is that brahman is not knowable - it involves only the element of eternity (knowledge, the process of knowledge and the knower are homogenized in brahman - try and understand that

)
paramatma realization (seeing god in the heart of all living entities)involves eternity and knowledge (the knowledge element is knowing the distinction between the consciousness of god an d the consciousness of the living entity, even though both consciousnesses are eternal)
bhagavan realization involves the added element of ananda or happiness, since eternity and knowledge ar ethere with the added element of service, or loving reciprocation between god and the living entity bereft of the dysfunctionalism of ignorance (associated with material consciousness)
all three, bhagavan, paramatma and brahman , are nondualistic aspects of the absolute - much like the sun disc in the sky, and the diffusion of sunlight are the inseparable qualities of the sun