I think you'd have to ask the activists. It's something some have noticed being used recently, and although it's not an entirely new concept, I've never seen "fake trans" being acceptable to activists. Maybe others have?
The "hoax-gender " seems to have increased in popularity at the same rate that crime by "trans" people has been increasing in reportage (using quotes here to indicate that it seems a lot of trans-identfied people don't consider any criminal to be really trans). It can no longer be hidden, so must be explained away.
It doesn't, however, sit entirely comfortably alongside "I am trans if I say I am" and we still see activists on these threads caught out by this. Just another sign of the ideology becoming more of a Hydra, in my opinion.