i mean i'm not gonna be a huge stickler on this or anything. mostly it's just, could this code reasonably be used for two games? if it's something you've already written then i don't want to force you to retype it to maintain the purity of the jam or whatever. but if you're essentially reskinning an existing game then that's not so much in the spirit of a game jam
but even then it's hard to make a hard and fast rule — like i've suggested zdoom as an engine before, and you could do plenty there with just a map, not writing a single line of code