Pretty sure that only applies when 13" rotors are used.
Per the info I saved for the old AR kit, the 11.75" kit with Viper calipers could use most 15" wheels. The DoctorDiff kit is different in that it uses a different kind of bracket (machined aluminum vs. flat plates with spacers) and a different rotor so pad spacers aren't needed, but I suspect the caliper location didn't change so I would guess a 15" wheel still works. But it isn't spelled out on the website, so if it where me (and it mattered), I would contact Cass to make sure.
BTW, when I say 15" wheels work, that doesn't mean every 15" wheel will work. The caliper can hit the spokes of the wheel even though the hoop doesn't, so the wheel should still be verified.
