Yeah, I think by active they mean posting rather than boosting. I agree that's not clear, I guess if they did it the other way round some people might complain that it doesn't ignore boosts?

Also, some people may not boost or post but are still reading posts and following accounts.

By the way, just out of interest and please feel free to ignore this question if it's too personal, is there some particular reason for pruning follower lists to remove inactive accounts?

I'm guessing it's only looking for new posts 🫤

I went through that list manually for that reason, and posted if anyone I dropped really is there they should totally follow again. It's not perfect, but nothing is in this world.