To be fair, Peyton's never denied that HDH had been shipped to his wife. His position has conveniently been that his wife's medical history is off limits.
If you fast forward to the 41 minute mark here, you can see exactly what Charlie Sly claimed about Peyton:
Watch the full Al Jazeera documentary that links Peyton Manning to a doping ring
To summarize, Sly claimed that:
1. He'd been an intern at Guyer
2. He'd met Manning
3. The Mannings would come in after hours and get treatment.
4. The Guyer Institute sent HGH to Ashley Manning.
Manning claimed that Sly's account was a complete fabrication. Well, he already acknowledged that point 3 was true, and his team has now acknowledged that point 4 is true. Al Jazeera (and the Guyter institute itself) confirmed point 1. So what, exactly, was the lie? If everything that Sly said is true, then why was Peyton calling Sly a liar, so the point that he was threatening to sue for defamation?
At this point, it seems like the best he can hope for is the claim that Sly was lying about having met him, which is a tangential, borderline irrelevant point if all of the other allegations are true, and I don't think Peyton would know if that was true either way since there's no way he remembers the name and face of every intern that a clinic that treated him has ever had.