What other theories should Kirk have considered, that fit the facts as they knew them at that point?
That they are inside some mind trick or under the influence of some malevolent alien? It happened many times before that episode.
They always ran into things that drove the crew mad, delusional or simply degenerate.
The fact that Kirk somehow just guessed the right answer for that situation seemed kind of far-fetched to me.
Even if we count the Talos IV incident (which didn't involve this crew), and maybe Trelane, I believe, at this point in the series that "always ran into things" of the calibre to look like a parallel universe is probably overstating it.
Indeed, at this point, Kirk had practical experience with alternate universe hopping to know it might make you dizzy, so he asked the crew.