The argument is that due to the Virgin Birth He would've only received X chromosomes from Mary and thus would have to be XX at birth.
By that logic, Jesus would've only had a single chromosome, unless Mary had given a second X by a genetic fluke (which can happen rarely, see Klinefelter syndrome). Thus Jesus would likely be born a woman with Turner syndrome, which doesn't seem supported by the historical record.
It's a cute enough idea, and I like the idea of a trans prophet, but it misses the mark when it comes to the details like this.