Except that they know exactly where the ball was placed pre-snap and they almost always discuss it before throwing that particular flag. It would have been a pretty easy determination.
The referee should be throwing the intentional grounding flag. His only responsibility is the QB and site line past them (OL sometimes RB). The referee should never ever be following the pass, QB is #1 priority.
It should be on the down judge or back judge to determine if a ball could be reasonably caught and to report that information into the referee. That’s why we saw the discussion.
If it were me officiating, I report it as no realistic chance of completion, the ball was thrown into the stands. It becomes the referee’s call as to whether the QB was outside the tackle.
We generally give three to four big steps at the hs level. That was not met, referee should have banged them with the flag.