That's actually really interesting. The ref didn't slightly go for the flag....he made a very fast and violent move to yank that thing out of his belt and throw it 10 yards....and then all of a sudden said "nope, better not." Why?
My guess is he realized the tackle was being made for a loss ANYWAY, so the penalty did not impede a defender from making his play...therefore....just let it go.
That should NOT be the rationale, but that's my guess as to the ref's thought pattern.
That's just horrible. Though we don't know what he's looking at or thinking, TBH.