I would think if you list them and include a reference from where you got them, you should be fine. And you wouldn't necessarily have to add the external asset's source into your project because it's not something you wrote yourself and with the reference to it everyone can get it themselves.
And freely available assets including their source is a bonus, but by the rules (as I understand them) not a requirement. The requirement is for the asset to be available for free (and not having some entanglement in its license that would prevent you from putting it into the source that you publish with your game).