I'd be very interested in a study that found that, say, a white woman produced more and better milk than an Asian transwoman, and that the difference was down to their ethnicities. Let me know if there is one.
Again you seem uncessarily combative on this - SES, ethnicity, weight and I'm sure many other things will be confounders. I'm not definitively stating anything about what demographic groups produce "better milk".
You want scientific evidence to be robust so that criticisms can't be used to invalidate it.
If you found that milk from TW was unsafe, you want to be sure that it couldn't potentially explained by other things (i.e., confounding or bias), so that it could be used to inform policy- for example legisaltion against TW donating milk or whatever.