I don’t think it’s cheating or contrary to the spirit, since zero day’s don’t come along every day, and people could have prepared exploits in advance for any of the platforms.

I do wonder though what they would have done if 5 people had been standing there at the start of the 2nd contest day, each with a usable exploit against the same machine. Who wins, do they split the money?

He must have had a hack prepared.

That’s cheating, it may not actually be cheating, but it is against the spirit of the rules.