No, he was discovered because his videos were on the internet. Scooter Braun misclicked on a link and a video of Justin Bieber played, that link could have been to any website. The value there was the internet, not Youtube. If you want to argue how Youtube plays into the success of Justin Bieber you should look later in the timeline, Youtube was a valuable part after he had gained traction, just like radio and television, it was a platform for distribution (not sure if that's the right word) of the brand.