I've seen folks answer that it doesn't matter what gender a trans person would be if they weren't trans, whether it was their imposed state, or their desired state. I think it is important how we have this conversation though, because I see a lot of folks treat the "goal state" of any given binary trans person as, in some sense, inherently cis - and by that I mean that trans people's genders are conceptualized as inherently static and as conforming to cis presentation of gender.
Like the idea, "if a trans man were not trans, he would be a cis man," despite being well meaning, entirely erases the traversal inherent to transness. Especially for trans men who love not being cis, who fulfill and represent a manhood which is not conformant to cis ideals, nor inherently based on the identities of cis men. This same principle stands for the rest of the trans spectrum, and especially for nonbinary folks who have no cis counterparts, trans men have just been the example in the posts I've read.
Let's say I am residing in one city, and travel to another, if I had never traveled, I would still be in the previous location. This concept is important, trans advocacy, as far as I am concerned, requires that we grow beyond the idea of gender as static. In my metaphor, if we argue that I actually inherently reside in the second city, while that can be my personal experience, and I may personally choose not to acknowledge my previous city, is not useful to pretend I never moved when defending my right to reside somewhere besides the place I was born. That is to say, holding that transness includes traversal of states and identity, whether internal or external, is essential to trans liberation.
The utility in "born this way," rhetoric has especially run short in the fight for unconditional acceptance of transness and queerness at large - because to think all queer people were simply, "born this way," is to believe that queerness is contingent upon permanence, further dependent on the innateness of their identities - which is cissexism. The idea that gender is innate and unchangeable is core to patriarchal enforcement after all.
We use the argument of innateness to stave off coercion, or conversion, but the coercion and conversion are the problem! The idea that any identity or state which is not innate can and should be subject to molding and manipulation, is the fucking problem!!