I do not know if Dr. Falconer is right, but I have speculated much the same thing.
The scriptural evidence for theosis is quite strong, particular in the Doctrine and Covenants and the Pearl of Great Price, but it can be found even in the Bible. My understanding is that thesosis is an acknowledged concept in Greek Orthodoxy, though not identical with our concept. For that matter, while I am no expert on Catholicism, it seems to me that the Catholic concept of partaking of the divine nature is not so different from the non-caricatured version of the Mormon doctrine. If there is a critical difference, it is probably that the Mormon version implies participation by deified humans in God’s continuing process of creation, while my impression is that in Catholicism creation is “a done deal.”
The scriptural evidence for “As Man now is, God once was”, seems mighty scanty to me, the more so the more I examine the question. Falconer gives some pretty good explanations of why that view caught on, and why we need not necessarily feel obligated to cling to it.
On the other hand, the infinite regress of Gods implied by “As Man now is, God once was” solves certain cosmological problems. So I remain undecided. Which may be just as well: Not only do we not know the answer, we may at this point be incapable of understanding the answer. “Eye hath not seen, nor ear heard …” Which could explain why more has not been revealed about it.
Of course, another explanation of why more has not been revealed about it is because obsession with mysteries, to the detriment of learning correct character, is hardly a uniquely Mormon problem. Kaballah, anyone?