Howdy! First time participating in mini jam, just wanna check I'm understanding this rule correctly.

"You may only make your game within the time frame, we do check. However, updates are allowed after the game has been submitted"

I've been very stuck for time this weekend and haven't been able finish my game, does this mean I can submit a broken/unfinished build and then update/fix it tomorrow?

in my opinion, i think yes. but it will only affect your score on the jam.

You are allowed to update, as long as you keep the original jam build available for download too.