There's a difference between flagging membership and flagging safety or allyship.
You can't flag safety in a meaningful way. Allyship is similar.
You can _absolutely_ flag membership and this technique is very, very old and very, very powerful
Both can be easily co-opted, but that's not the point. One puts your skin in the game in a way that the other does not. One says "this is who I am" the other says "this is how you should see me"
I suspect many disagreements come down to this perception