Yea, I think this is the biggest thing. Most "real" celebrities can still post on Twitter or whatever. If Elon Musk or Lebron James or Barack Obama posts something important, I'll probably see it wherever they decide to post it. They could just yell random things at passersby on the street and I'd probably find out about it if it was interesting enough.
The value of Twitter a lot of the time is less the big name Blue Checks, but the long tail of niche blue checks. I bet a massive amount of people on Twitter have no idea who Paul Graham is, or Popehat, or Marco Arment, or even dril, but they all have some number of people who were mostly on Twitter because of them primarily, so if they and their friends all move to something else, their fans will follow.
In some ways, it's not too different from TV hosts or journalists of note. People will watch John Oliver on whatever TV network he's on. For certain niches, no one cares where they write/speak as long as it's not so insanely paywalled or otherwise cumbersome that it's a problem to follow them to it.