My personal guess on what happened was that Sly poked around in old medical records for Peyton and his wife while he was bored one day at the pharmacy, saw the HGH there and decided to weave a true fact into his hustle about how PEDs worked in the clinic. That being said:

The lawyer who visited the Guyer Institute did not remove any records, Fleischer said, and Manning’s investigators in no way influenced or coerced Sly into issuing his recanting statement, which he recorded Dec. 24, the day after they had questioned him.

That sounds sketchy as hell. Manning had to know that there'd be an NFL investigation with allegations like this and decided to preempt that with an investigation of his own. His people went to the house of the prime witness and had discussions that led to the witness spontaneously deciding to recant.

If the NFL is going to throw suspensions around over a phone that they don't have the right to actually look at, what should their response be when someone runs around (presumably) unsupervised talking to key witness? Did the PIs threaten legal action or criminal action (one initially claimed he was law enforcement before clarifying that he was just a PI when law enforcement showed up)? Did they say he either made a criminal HIPAA violation (releasing personal medical information for personal monetary gain, like getting another client, could be 10 years in prison) or just lied a bit (harmless low dollar value fraud, no big deal) and he should clarify before the story comes out and the police come looking? Were there any other threats (lawsuits are expensive, even if you're honest) or inducements made?

I realize Peyton wanted to make sure his name stayed clean, but it was a mindblowingly bad decision to go off on his own instead of involving the league at that point.