I did write down some "typical reasons" for de-federation in our Wiki: https://wiki.f-hub.org/books/slrpnknet/page/lemmy-instance
But it is intentionally a bit vague and left open for interpretation as there will be always edge-cases and people trying to game rules. So no, I don't think it will be ever possible to have "clear circumstances under which we would or would not defederate", its always a bit of an individual judgement call related to both our specific community here and the health of the wider Fediverse.
In the specific case of Threads, I think it would probably not matter so much for our specific community here, but the threat to the wider Fediverse is very real and warrants a coordinated attempt to keep them out as best as we can.
And "denying potentially millions of people from experiencing the fediverse" is just not the case. They are free to make an account on another Fediverse server that is not run by a known bad actor like Facebook.