Bi erasure isn't always about saying bi people don't exist. Just that they're an 'other' that isn't quite gay and don't really belong in the same spaces as gay people because they don't experience 'gayness all the way' as some have put it. It's about minimizing their experience and saying they can be more comfortable and less part of a rigid category therefore are less relevant or occasionally less valid. (This is something I've personally experienced.)
But transmedicalism is trying to use dysphoria or 'medical condition' as an excuse for validation which isn't, imo, all that validating, nor a complete coverage of the trans experience. Kind of like seeking out the 'gay gene' that will never be found. And will never be important to validating the experience of gay people. But don't take my word for it. There are trans voices talking about this too.
(Actually this whole video is great but this particular segment talks about this specific topic.)