The only explanation is that Hakai falls under the trope of being an energy based attack that we see riddled throughout shonen manga and comics in general.
You see shit like that in Dragon Ball all the time, where a character gets hit with an energy attack that could decimate a planet, and not get pushed back at all, but then they get kicked and are sent flying through like 5 mountains.
There is no need to dig deep into it. It's just tropes. The proof of the strength of the attacks are judged solely on the damage caused. Not by how much it can send someone flying.