Aye, I had similar thoughts, cause it is not written specifically in the rules - but your clarification is clear and should be added to the jam description somehow, if possible.
From my understanding, something like: "Synty assets are to be visually predominantly Synty, be clearly in the focus and spotlight of the game. 3D assets can ONLY be from Synty. Other filler assets which you created or have the legal rights to share and distribute for UI, 2D, Audio, Fonts, ... may be added, as long as they are not taking center stage and the focus away from Synty.