But was it a an original-fic-only community, or fanfic communities performing fandom differently? Because I'm in a fandom where original fic co-exists with fanfic (same people, same boards, same terminology, same themes, same clichés and tropes, etc, etc), and they would likely be people who would post it to ff.net if they were allowed.

I can't say that they won't be many many fics so. I don't know. German fandom apparently has many many original fic going on. My current fandom has a very modest amount, but it still exists. I'm still pretty doubtful the A03 would attract non fannish people. Like, really really doubtful.