Well if I say: "I will build a hot-air balloon" then it's reasonable to interpret that as agreeing to the rule "I have to build a hot-air balloon", so if I don't, I'm cheating.

It doesn't, because rule contravention is not the sole sufficient condition of cheating. Cheating involved 1) breaking rules that 2) others are following for 3) advantage whilst 4) disguising the fact.