We don't know enough to say if people are transgender because their hormones are imbalanced, but we do have reason to believe that transgender people tend to have a distinctive hormone balance even prior to any hormonal therapies.

For example, female → male transitioners have like a 2x - 3x higher incidence rate of PCOS before any therapies. Obviously there's something different going on.

The EDC question is also related to prenatal hormone exposure, which is not related to your anecdote about pre-therapy hormone testing.

In any case, the theory being put forth here re EDCs is not that every (or even most) transgender people are such due to EDCs and resulting hormone imbalances, but rather that even a small change (read: compression of variability) in hormone distributions across the entire population would produce all sorts of effects like everyone being slightly more androgynous. If that's the population-scale effect, you'd presumably expect more people to slide each way across the binary designations we tend to use for gender.