You are referring to the "down by contact" rule. True.
I just don't know if that was the call there or not.
Not just down by contact, happens on incomplete passes with further action too. The best you can get on a challenge with something like that is to get them to call the pass complete, but nothing that happened after that (ie after the whistle blew) will be added to the play. Really it was probably Seattle that should have challenged, given when the whistle blew they actually probably would have benefited from a call of completed catch and fumble. It gets hairy on challenges, especially midfield challenges (challenging an endzone or falling out of bounds incomplete call is pretty cut and dry since with either call the play would then end the only question is what kind of end should happen, midfield catches that might or might not have continuing action are rough) but there's no way Pittsburgh would benefitted from a challenge.