I'm not one of the moderators, so I might be wrong, but my understanding is that because they want us to upload our source files, we can't be uploading any files that we're not allowed to share with other people. So the things we get from Unreal free each month, we paid no money and got a license for, but we still can't share those assets with other devs, so they're not something we can upload in source files either.
Edit: You're right, they didn't say we have to upload the source files this year; that's great, it felt like an outdated step that probably made sense in the early days of game jams but always just added extra steps to an already stressful time period.