it says theme is hidden til submissions are in, but then how am I meant to know what to make, I need to know the theme to get started
It doesn't make sense to me either. From a jammer's perspective it looks like:
1) Pick a theme
2) Put in the work to make a game - and even a little lame game takes non-trivial amount of time and effort
3) When the actual theme is chosen after-the-fact, probably be rewarded with disappointment because the actual theme is very likely to not be the one I've picked.
?!?