It'll be interesting to see more research come out on this. They go through a pretty extensive list of substances, not just pesticides, that disrupt sex hormones. It seems like a lot of synthetic estrogens have been introduced to our environments in the past few decades.
The susceptibility point seems important, because cultures that still emphasize sex differences don't seem to have the same trends of trans identification, although they presumably do have the same physiological trends like lower male testosterone.