Why can't there be breastfeeding groups for bio women and also separate ones for transmen? It makes sense because both groups have different needs, completely different problems.
Because transmen would be welcome at groups, biologically the advice would be relevant, and although they may benefit from extra emotional support for different reasons to others, there wouldn't be enough in an area to make meaningful support groups. However, in that scenario you'd hope others in the group would use preferred pronouns, and the additional unique support could hopefully be sought online or through other avenues. As long as they didn't expect the entire language to be changed for everyone, just for them.
Transwomen on the other hand, aka men won't benefit from support groups, because the advice won't be relevant. General feeding groups, maybe, but as a formula feeder BFing groups would also be irrelevant, for example, and someone going in and asking for the nature of the group to be changed in case they were offended would not be welcomed. Not comparing FF and chestfeeding btw, the later is not in the best interest of the baby, and there's a lot of ??? around it.