I haven't rewatched the game, but don't recall if Kosi was held and/or if there was still momentum. IMO, the issue was that Kosi tried to get up again after hitting the deck and then got pushed over the sideline. If he stayed down, we continue attacking their line...And Kosi pushed over the side line.
He was on his hands and knees with a hand in him. The tackle was complete. And THEN the cover defender made contact and in a second movement shunted him over the sideline.
Without the second tackler after the tackle was completed he would have stayed in play.
Now some of these are 50/50 calls. But they are not 50/50 when 100% of them go against us…