The way I've seen it phrased is slightly different, like you're not a real femslasher if you ship femslash in majority-slash fandoms, like MCU. Honestly there's a lot of hubbub about "real" femslashers in general, going all the way back to complaining about people just pretending to like femslash pairings for the SJ cookies. I have no doubt there are some people out there like that, but the obsession with them means that femslash shippers have to pass some kind of test to prove they ~really ship f/f, and none of the het or slash shippers ever have to do that.