You have an example a minute later when Butler collides with the Seahawks receiver while playing the ball with no penalty called.
First of all, Butler gets the ball. You come up with the ball, and seconds later, it's clear why you made the play... it wasn't hitting someone defenseless, it was making a play on the ball. Look! You have the ball! No contest.
Same would apply had Butler interrupted the play, but dropped the interception. Clearly he had a play on the ball.
Now without even going into tinfoil hat mode, the guy is flat on his back, with the ball not in his possession YET by the time Harmon is going by. Harmon's play, if he can make it quick enough, is to jump on the ball as it hits kearse's body. Only if he doesn't have the timing to get to the ball, to at least tip it, he's just nailing a receiver flat on his back. Now he says later he thought the ball hit the ground, as did everybody else... so if that's true he isn't even judging on that merit.
But let's say Harmon is misremembering and in the moment he understands that the ball hit Kearse not the ground.
He doesn't get a piece of that ball, he's flagged, period.
The beauty of the Butler play is, what do you lose by going for the INT? I guess you could conceivably be hit with half the distance/loss of down, but it's a much less ambiguous moment. That and the possibility of being flagged is so low b/c you have the right to go for the ball... and the proof would be hands on the ball, if not INT.
So while we can all fight through Easter morning about whether Harmon had a play, but saying that the Butler play was on equal footing doesn't wash for me. I think that if Harmon leaps into that play and there's a flag, we're all sitting here arguing about whether he could have jumped clear of the play.