You don't seem to understand what you're quoting.

Executive function does, of course, continue to increase throughout early adulthood.

If being trans was a matter of a boy/man deciding on a whim to permanently cosplay as a girl/woman instead of as teenage mutant ninja turtles, and this was done without supervision, then we might be worried.

However, if the feelings are deep, lasting, and troubling, you don't need any more advanced executive function to understand, any more than you need advanced executive function to know that you love ice skating or want to spend three hours a day practicing the violin. And if it's done with careful supervision to tell the difference between genuine distress and a silly and transient impulse, then as usual with major life-shaping events with children, they're checked and validated by adults who fill in the executive function that the children might lack.

Anyway, the problem is that your arguments are assuming that trans people aren't like (at least some of them) say they are. But the psychology of gender dysphoria is as well-confirmed as practically anything can be that isn't really externally measurable--it's certainly consistent, correlates with psychological distress, and so on.