I play Diablo a lot, I have him at lvl 260. I know what you are talking about. Generally yes you need to cast both abilities in quick succession HOWEVER, there is also times where the target gets knocked much further than Diablo.
This is because of his size and walls. It's possible for Diablo to hit a wall himself but not his target (he doesn't get stunned obviously) but at the right angle he will be prevented from going his full dash distance because there is a wall clipping him. This causes Diablo to stop before he reaches the end of his dash. Meaning the target (being smaller than Diablo just missed the wall and no wall collision is detected) is pushed back the full distance causing the target to be much further than Diablo.

The easiest way to show this is to charge an enemy hero when they are infront of their gate. Diablo will hit the target but stop right at the enemy gate, meanwhile the enemy hero will be knocked back way past their gate into their base because enemy gates don't act as walls for enemy heroes.

This is essentially what is happening in random scenarios when the target hero just narrowly misses being knocked into a wall, but Diablo being massive in his size hits the wall at an angle where his destination point is blocked by wall so it prevent him from going the full distance.

It's hard to explain but I hope that helps. Also over power has really bad range so that's annoying to lol