Remember again that this whole claim – that Cohen is willing to testify to the existence of such a meeting – is based upon a leak, and Cohen’s lawyer says that their side is not responsible for the leak, although he did not deny that such testimony was Cohen’s to give. So at this point the news story is this: Giuliani is denying the existence of a meeting that nobody has said existed in the first place! Such is the nature of the voracious 24-hour news cycle. But let’s consider the imaginary story, assuming that Cohen really did make the accusation.
First of all, there’s kind of a philosophical point Giuliani is missing. How can there be witnesses to something non-existent?
However, he’s not really talking about “witnesses” as we understand the concept. According to the leak, Cohen claims to have been there when Trump Jr reported the Russians’ offer to Trump Sr. Giuliani’s claim that there are “witnesses” is his weaselly way of saying that the two Trumps deny that the meeting happened, which is not exactly breaking news. They have been denying all along that there was any communication between them about the Trump Tower gathering, so IF they were lying, and IF Cohen is accusing them of lying, Giuliani is merely repeating their lies rather than offering any rebuttal.
However, keep in mind that they may not have been lying and/or Cohen may have made no such accusation, leaving Giuliani simply trying to piss in the wind without even feeling the urge to go!