Maybe I missed something, but I didn't get the impression that Glenn was interviewed for the film. I hope I'm wrong, but the reference only mentions that these folks that hung out there. The way this reads is that if Glenn was interviewed, then so were all of the others that were mentioned.

Not only did "The Troub" host their (Taylor and King) first appearance together in 1970, but it served as the showcase for all their singer/songwriter friends composing, writing and living in nearby Laurel Canyon. That rustic, hilly terrain, winding between the suburbs and the city, became a kind of musical campus, peopled with the likes of David Crosby, Jackson Browne, Joni Mitchell, Glenn Frey, Bonnie Raitt and Kris Kristofferson.