Not a poor choice at all, once the ref blew his whistle and pointed to the ground the play was done, nothing about a red flag could have changed that. Once play is ruled over you can't challenge to get anything that occured after that point, because they know players stop when the whistle blows and it would be unfair (and possibly unsafe) to reward a player for not stopping when the refs said the play was over.
You are referring to the "down by contact" rule. True.
I just don't know if that was the call there or not.