If philosophy were to create concepts, then the question immediately arises of how one would determine whether these concepts were created correctly, or truthfully. The understanding here is that concepts are universal, and not contingent on anything, including on how they were "created". Instead, I think Deleuze should have defined philosophy as something like "the discovery of concepts, a discovery that, once accomplished, reveals to us something we had already known."
Full disclosure: I'm defining concept not in the regular, everyday way, but more in line with Hegelian, Idealist way--der Begriff.
The rest of your comment seems predicated on a Platonic idea of concepts; which is definitely not in line with how Deleuze thinks about things. His ideas are very much not in line with the typical way of thinking about concepts as universal things we discover.
Edit: I should note here that I’m not an expert on Deleuze or Platonism, but I would probably say that Deleuze’s skepticism makes more intuitive sense to me.