Thank you for your question; I imagine others may have similar concerns around this when participating as a solo developer.
Are ALL assets that are not created during the week of the game jam considered “sourced”?
The spirit of the MegaJam is to create something new that relates to the theme (revealed upon kickoff); both of those things typically mean that completely original works will have a broader opportunity to create something unique based on the theme. In short, yes anything made before the event would be considered sourced and must be noted upon submission, but that doesn’t mean you cannot or should not use sourced assets; there is something to be said about implementation and other areas that directly impact the rest of the judging criteria.
There are also special modifier categories with specific criteria that might appeal to you.
I want to stress, make sure you have fun, and don’t be too hard on yourself in the lead-up to and during the event.
I hope this helps!