Serious foul play would mean an advantage would not be allowed to go on, however a red card offence could be allowed to continue through advantage as long as its not serious foul play.
not sure where you get that from. as I quoted earlier from the laws of the game
Advantage should not be applied in situations involving serious foul play unless
there is a clear subsequent opportunity to score a goal. The referee must send
off the player guilty of serious foul play when the ball is next out of play.
A player who is guilty of serious foul play should be sent off and play is
restarted with a direct free kick from the position where the offence occurred
(see Law 13 – Position of free kick) or a penalty kick (if the offence occurred
inside the offender’s penalty area).
a goal was scored so it's pretty clear there was a goal scoring opportunity so if it was considered serious foul play the ref must play on, give the goal and send off the player. he didn't do this so I'm assuming he didn't consider it as serious foul play
I think (but not sure) that you're saying that they never actually apply the law this way despite it being pretty clear. you may well be right, there are certainly some situations where refs generally do something slightly different to what they should do according to the letter of the law either because it's just gradually become accepted or because they're given certain guidelines