Oh I see your point- that the comet had been set in motion many years before.
Okay, if we believe in miracles, (which I do) He could have diverted the comet had they repented at Noah’s proclamation of righteousness.
I believe we usually choose the harvest we reap.
Or the comet could have been on a non-collision course unti God diverted it INTO a collision course.
I don't see this theory as contradictory to scripture at all.