No, 5 purple is the maximum if using the standard difficulty scale. An opposed check is going to break that by default since it's not using the standard difficulty scale.
I see, as I suspected... Not very elegant from their part, but ok.
Since it is already envisioned in the rules that PCs may face difficulties of 6 purple dice (through opposed checks), I don't think anything breaks in the game if this "standard difficulty scale" extends to 6 purple dice (or more).