I note the article is careful to say it is the "structure" that shrank, not the materials themselves. It will be some process involving bending, for sure. What they have done, I think, is to design an ingenious structure in which the net effect of the bending is such as to reduce the overall dimensions of the the structure, which is far from solid.
If one were to immerse this structure in a fluid of known coefficient of expansion and then heat it, my guess is one could measure the displacement of fluid and thereby the expansion of the materials comprising the structure.