A few thousand out of millions is not a lot. The fact that it took them until now to leave, just a few weeks after Musk took over, shows that it's not anything policy-related and motivated entirely by the removal of insiders from positions of influence within Twitter. Now they want to go infest the Fediverse. I think they'll find they're just as unwelcome there as they are on Twitter.
A few million, in contrast, is a lot. I partially agree that there were already several reasons to leave and that only leaving now means many will return. However, as someone who's been on the Fediverse for years and never found Twitter even usable, I must say the following: I don't really see how those users were in positions of power or influence, not do I see how they would be an infestation to the Fediverse. I also have not seen much unwelcoming content to the Twitter refugees in my area of the Fediverse. Though it is very hard to make judgments about the Fediverse since it's not even centralized or monolithic.
It's around 5000 per hour, well over a million new active users this month.