This question is less for myself and more for a teamate I want to recruit. He's worried that since there is no theme that people can submit games they've been working before the Jam (in secret) to have a head start.

So I'm kind of curious myself: how is the integrity for the games determined?

I have a hunch on how they check stuff like that, but I'm not going to say it in fear of exposing it and people figuring out how to get around it

Ultimately I think the theme this time around does exactly what Nick said, especially if you look at how points are awarded. Gotta show what you can do, not how much time you spent on it.