Next time you should make the wording for the rule more clear, because

Only submissions for this jam are allowed

sounds a lot like

The lifespan of an onion is determined by how long it lives

: one could think

¡Of Course only submissions for this jam are allowed; otherwise it wouldn’t be a submission for this Jam!

in the same way that they would think

¡Of Course the lifespan of an onion is determined by how long it lives; otherwise it wouldn’t be the lifespan of an Onion!

. much better is something like

Submissions must be just for this jam and not for other jams

.