Substack's founders have been vocal about supporting free speech[0]. So it makes sense to me why they wouldn't want people holding the exact opposite views to the point of quitting.
There are reasons to not want to work for Elon Musk other than disagreements over free speech.
Yeah I think that is completely fair but the tweet states "If you’re a Twitter employee who’s considering resigning because you’re worried about Elon Musk pushing for less regulated speech… please do not come work here."
I find Musk vile, but I haven't seen many other reasons cited; all I hear is that he was threatening to let the "actual nazis" run wild.