It Didn't happen.
I can summarise the paper for those who didn't read it:
Man with penis and testicles takes female hormones (and several other drugs)
Impregnates woman
Orders a galactogogue drug illegaly
Claims to be pumping and to be producing milk
Claims the pregnant soon-to-be-birth-mother doesn't want to bf
Has HIS blood levels monitored by two researchers, one a female doctor, the other a TIM nurse, both of whom are transgender focussed
Baby born
Claims he is the primary food source
Researchers write up his claims as fact with no effort to validate them
Tell me, if you were about to claim a world first, do you think you would
a. Make sure you had evidence for such a remarkable claim
b. Make sure you reported said evidence
c. Make sure you covered any controversial angles such as, oh I dunno, actual drug levels of the carcinogenic drugs in the breastmilk?
Read the paper, if you can.
It is most revealing what isn't said
I believe this baby was fed via another source and this is a complete fantasy. The researchers should be ashamed and should be challenged to provide all details regarding how they validated this claim.
It's a fraud.