I'm only going by what observers have said and a logic I have agreed with. Your point is fair, but its not a legitimate challenge in my book.From the LOTG glossary: Challenge - An action when a player competes/contests with an opponent for the ball
That's the entire definition. And nothing in there about the ball having to be in play. The same applies to the glossary definition of Serious Foul Play. So I'm disagreeing the the initial premise of your statement - even if the whistle has gone a split second before the kick, that doesn't stop it being a challenge for the ball. Which therefore means SFP is still on the table as an option.
As said earlier, horse for courses, player still gets red and associated ban for it.