Uncatchable balls negate pass interference but have no bearing on defensive holding as far as I know. The ball doesn't even have to be passed for there to be defensive holding, just holding. It doesn't even have to be an eligible receiver that is being held. Sometimes they will call it on a defensive lineman who is holding an O lineman so that he can't get to the second level to block a linebacker.
The frustrating part about them calling off the defensive holding is that Brock made the Smart play and just threw the ball away to live to play another down. Picking up that flag was inexcusable.