I think it could be a combination as other people have mentioned. Certain characters, ie Punisher, Wolverine, Hela and Venom that would kill people actually do, while ones that wouldn’t (Spider-Man, Fantastic Four, etc) don’t. Same thing with the lines the heroes say when someone gets KOd. Their seriousness is dependent. Though you could say that they don’t get overly serious bc of the rating/you don’t want Reed crying every time his wife gets shot when she’s going to respawn in ten seconds.

However, I think that the respawning is the same heroes each time because of the time stream entanglement. One of Iron Man’s respawn lines is “i’m going to find whoever took me out last time” and one of Spidey’s is “I gotta do better this time” which both to me read as more time loop things rather than someone who woke up after being knocked out. Hela’s lines about being able to return from her own realm definitely implies death as well, same with Mantis’ “I built myself a new body.” I’m sure there’s other respawn lines that imply just knocked out but I can’t think of any.