Does it matter if I add cornstarch to my sauce instead of pasta water?
Flavor, that is all. The water left over from boiled pasta tends to have the flavor of the pasta, and a salty flavor. If you season your pasta water with salt, and I always do, why not bring some of that flavor into the sauce you are preparing? Better than just tossing it all down the drain.

Does it really matter? No, not really. But, if you want to use some of that water for something, add your starch to it.