I was under the impression that they said there would be no Pixar movies featured because when the first movie came out, Pixar was still an independent entity and retained the rights to its characters.

Last I checked, Disney bought out Pixar. I don't think the legal issues are there anymore, so the boundaries which originally restricted Pixar movie involvement are likely now removed. Pixar owns the rights. Disney owns Pixar. Therefore, Disney owns the rights. It's simple transitive property!

That said, I'm not sure it's likely, but it seems like a logical step for the series as far as new material goes.