That's been true for 40 years. Jimmy Paige got his start as a session musician, partly because guys like Pete Townsend couldn't get the hang of that newfangled electric guitar. Recording time was very expensive in the mid-60's -- hence you brought in the hired-guns to lay down the guitar tracks quickly. Getting it right on the first cut was the goal. Incidentally, you won't find Paige's name on any of The Who's albums. He got paid a premium to play uncredited. This preserved Townsend's reputation as well.
I have never heard that story, but it explains why Paige was chosen by Moon and Entwistle when they founded Led Zeppelin.
Now I certainly know of Page's tenure as an in-demand session player, even at the tender age of 17. However, saying that he was responsible for many or even some of Townsend's recorded licks.............sorry; don't buy it. I'd have to see far more evidence to believe that. Townsend happens to be one hell of a guitarist (even if his lead chops are wanting).