That's a bonkers interpretation. Of course you can only "submit during the jam", that's how the website works.
I won't push any further since this is a prize-less jam, but the language is clear enough. What usually happens is the opposite: Jams that allow for projects started prior to the jam will go out of their way to make that clear in the rules.
Jams are not just about doing the game in the allotted time, but also about doing them "together". That's the spirit of any jam. If the author was truly misinformed about this, then pretending it's ok will just lead to a possible worse situation if he ever joins a jam with a prize.
(EDIT: It did not escape me that you were the only other person to submit right at the start as well. At least yours does look like it could be conceivably done in 5 minutes if you already had the text in mind)