Is it the "How do I upload and submit a game to Adventure Jam?" answer? It describes how to make and submit a game, but the intention there isn't to imply that that should be done ahead of time, but instead to highlight that the game submission side of things doesn't show the submission button/form until after the jam starts.
That said, I think creating your game page early is a good idea - leaving it to the last second tends to lead to panic, and people generally don't make the best decisions when they're panicked.
Making stuff that won't be in the game, but helps you explore and nail down the concept is fine, and it sounds like that's the kind of approach you're thinking of for your game page's assets.
Hope that helps!