Thanks for the catch, our addled brains didn't even include credit for the two of us!
Curiously, the jam rules as posted do not state anything about attribution, nor disclosure of prior assets in general, really. Just "majority of work must be during the jam, etc. etc." (Bit of a loophole there, in my opinion.) If you know of a more comprehensive, official guide, by all means point me at it.
Per alexdraws, the assets can be used any way the owner likes (which would include without attribution).
Looking forward to rating your submission in the next few days!