It's also really easy for Greenblatt to issue a denial.

That is, of course, unless it is true.

A denial wouldnt necessarily indicate that it is false (he has every reason to deny it, but lying is a risk) but the lack of a denial is very strong evidence that it is, in fact, true.

There is a very low cost to denying lies, so the absence of a denial (unlike its presence) is a very good indicator.