The Bluesky surge happened after a massive global election result and a massive grievance from progressives/leftists over Musk and how Twitter has become. Indeed, if you think lemmy is politically partisan - then Bluesky is no different.
The Reddit -> Lemmy surge happened because of some poor Reddit admin decisions. The scale of the events were on different levels.
When the next fuckup from Big Tech comes around, do you think that people will think about going to Mastodon/Lemmy/PieFed, or they will just look at Bluesky?
It would depend on the site origin of the fuckup. If Reddit fucks up, as a reaction - Lemmy would get many new users.