Male bodies and female bodies are much more similar than they are different. The gender affirming medications trans women take are for the most part, just the hormones that cis women have in the first place - they don't make the body do something radically different from what it was already programmed to do - they just nudge it from one built-in programme to another. Milk produced by trans women is made by the exact same physiological process that cis women do it - it's just that one would have happened anyway, and one was the result of a bit of prompting!
I wouldn't be surprised if, if you gave a sample of milk from a trans woman and milk from a cis woman to a biochemistry lab, they wouldn't even be able to tell the difference. So this makes perfect sense to me!