Yeah it's just a security patch, I don't think there would be anything phone specific in it? Samsung does roll out updates in staged waves... they must be waiting on the S21 line for some reason
Actually, thinking about it, I do believe I read in one of my security updates something about changes for my particular phone (like camera improvements etc), so maybe the updates are made for specific phone models. I think that's why their security updates are so much bigger than a Pixels... instead of feature drops, they add improvements in to the security update. That would also explain the delay.