Part of the problem is that "open source" is only clearly defined for software. Use in other context is metaphorical, which is neither clear nor precise enough to make good legal text. Perhaps it would be in order to ask CGTextures what they intended and if they consider release of a work including their textures under CC-BY-NC-SA compliant with or in violation of their license? A statement one way or the other from CGTextures would clear things up with no need for speculation.