I would like to ask, given that the theme of the jam was published long before it began, does this prohibit the creation of the project in advance? I didn't find anything in the rules regarding this, or is it an obvious thing at every jam?

Same question here. I already started my game  😬.  I think for game jams with the topic already given, you are allowed to start early. Just a guess, I don't know for sure

I don’t think you’re able too, if you were there wouldn’t even be a “Jam Start Date” it’d just be “Submissions open till (deadline)”

Oops, ok then. I guess I'll have to submit something else for the jam.

There is an additional theme its not just the shapes just FYI

From the page

"Additionally, a theme will be announced at the beginning of the jam period, and so you're encouraged to link that to your game as well!"