Doesn't matter if not all of them come over at once. There'll be enough people to ensure that threaded Fediverse communities can remain active, and as spez does more and more to flush Reddit down the toilet we'll be ready.
It's also a matter of how committed or active the people using 3rd party apps are - do they spend more time on average on the site, post more content, or are more likely to moderate subs? Perhaps. It seems to me that indeed people who know about and bother to use different apps probably are longer term users who care more than the typical reddit member.