P5k000 in that thread has a good explanation for why this happened:

The great part about AI is that it inherits its bias from the dataset. And most of these models are trained on mostly uncurated datasets of images from the internet, so its really showing us an accurate representation of what transgender programmers on the internet look like

It is obviously beyond the means of the developer to curate a dataset that big, and if Google tried it would massively backfire - telling a transsexual that they are so ugly that it would be detrimental to trans representation to allow them to upload a self-portrait wouldn't go down well.