Pretty much every big tech company was still providing severance for those who decline RTO.

Maybe they don't have to but there's sufficient murkiness that it's probably cheaper to just give the severance than go to court.

Which big tech companies provide severance for those who decide to leave because of RTO? It's not normally framed as "declining", either - you're simply expected to be there starting at a specific date, and if you're not then you're basically not performing your assigned work duties. You don't get to "decline".