I think, Blenderkit is a little special. The Add-on comes with Blender, but in the end it is just an interface for downloading resources from an external service. So, these are still pre-made models (either by yourself of from someone else) and I think it would be fair to treat them just like that when thinking about the pure/open categorization.
However, since your question was about textures: I still think that usually textures shouldn’t have a large impact on the pure/open categorization. There might be cases where textures are used in a rather unfair way (like mapping a photo on a plane and calling it a pure entry), but the texture’s source (blenderkit, local assets, camera, inkscape, gimp,…) shouldn’t make much of a difference.