Its not just youtube videos, I watch college football religiously and I've watched Baylor close to ten times over the past two years. I've seen more of Petty than Winston or Mariota and I saw an awful lot of those guys too. I don't know who measured what or how you measure arm strength, I go by what I see, and I guarantee you nobody will be questioning Petty's arm strength once they see him in a few preseason games, not if the coaching staff lets him air it out. Lastly, I do trust my eyes when it comes to arm strength, I've seen enough football to know the difference between a strong arm and a weak one. I know what a spiral looks like and I know what it looks like when someone can spin it with velocity. To compare Petty to guys like Danny Wuerful is laughable. I won't say Petty has a cannon, that would be a stretch, but his arm is about the same as Geno's and both guys can make every throw in the book, including the toughest throw of them all, the 15 to 20 Yard Out, thats the real test and I guarantee you Petty will have no problem zipping the out.
I'm not sure where I'm picking up the conversation but everything I have read speaks to how great Pettys arm is. Gruden suggested that he had the best arm talent in the draft.