I think it's meaningless. First, the claim is made with no criteria. Greatest in what sense? Technically? Most popular? Most well-known? Highest paid? We don't know. Our claimant never says.
What if criteria is stated, then the claim is made? Well, then I say that is the wrong criteria and I use different criteria and say my uncle Frank is the greatest magician ever, period. Am I wrong? Who can say?
So the claim that Blaine is "the greatest magician ever. Period.," has no meaning except to the person or persons who make the claim.
Oh, and the same applies to the claim that Blaine is a no-talent hack.
That's the nice thing about opinions. They're always right...For the person who thinks they're right.