When you use delete apps like powerdeletesuite or shreddit they cannot delete comments from private subs because they're hidden. So when those subs come back to normal mode, so will your comments. So all those people that used apps like that while the blackout is going on will have their comments appear again as subs reopen. Not saying reddit isn't also bringing comments back, (I wouldn't put it past them tbh) but a lot of it is just because of subs being private and the comments getting missed by the apps.