Descartes is widely considered to have nontrivial contributions to science and mathematics. Leibniz argued these have been exaggerated. In any case, philosophically I regard him as an arrogant fraud who really better fits the profile of an antiphilosopher.
Descartes simply refused to engage with the whole philosophical tradition, while replicating the ridiculous Stoic claim to possess a whole system of the world founded in certain knowledge. His abrupt dismissals of “dialectical subtlety” in favor of things allegedly simple and clear are mostly just bombastic rhetoric. The famous hyperbolic doubt seems to me but a rhetorical ruse to clear the way for a foundationalist revindication of traditional values that was in most ways far less sophisticated than the arguments of medieval scholasticism. (See expanded remarks on Descartes in Modernity, Again.)
Rereading Making It Explicit, I finally found a brief comment that better explains how Brandom somehow connects Descartes with some kind of revolution in normativity. It is pretty indirect. On p. 10, he says that Kant retrospectively read into Cartesian doubt an implicit requirement that we take responsibility for all our claims, and that we be prepared to justify them. This seems quite plausible, as a statement about Kant. Brandom offers his own account of the “notorious” failures of Descartes to adequately explain representation on pp. 6-7. In a nutshell, representation for Descartes is an unexplained explainer.
Thought for Aristotle and Hegel is in the first instance something shareable, determined by its publicly examinable inferential articulation. Cartesian thought, by contrast, is a private, interior affair. (See also Subject.)