As an example, SSB is developed on #SSB
because there's #gitssb.
I don't think waiting for git-OStatus or git-ActivityPub to emerge before developing ActivityPub is a good idea, and neither is staying away from the current tools of the trade for the sake of it.
Of course, there's more than one person involved who thinks using github rather than Free project hosting software is problematic, but there are many contradicting forces leading to that decision, and it's hardly unique for the Free world.
AP or OStatus support in #gitlab
would be really cool though.