We all know Law's 3 billion bounty is BS, so lets ignore him for the time being. Even if his bounty is well deserved, bro's nowhere near a top-tier, so we can't take him as a proof of anything. He certainly has top-tier potential even without having CoC though.
All admirals not confirmed CoC or CoC coating users, Big Mom isn't that great a CoC user although she does have it. Their existence alone is more than enough to confirm non-CoC coating users can reach the top. The discussion is about whether someone can do it without CoC coating or a DF, the answer so far is a no as there's no confirmed top-tier that doesn't have a DF or CoC coating.
We do have CoC coating users who can still fight at top-tier level without using CoC, but we don't have a pure non-CoC coating user who managed to make himself a top-tier without a DF. If Gaban did that, then he'd be the rarest of the rare case.