It seems to be an omission that they don't list providing the source as a requirement, since if you only have to upload the finished game one could hardly check if the assets are free or not.
Also, the rule always was they have to be free without a time limitation, so other students can pull the project well past the jam's duration and still be able to tweak and build it for themselves.
Thus, "free for this month" assets are off limits.
Regardless, unless it's assets you made yourself (and are willing to hand out to others) or are CC0 or similar to start with, even if they're completely free on the Unity/EPIC Asset store (or itch.io), you must not include them directly in your source upload since you're unlikely to have permission to distribute the asset files themselves directly! Instead, keep them in a separate folder in your project (that you exclude from any upload) and provide links on where to get them!