That's crazy af bro. I never thought about it like that, but yeah...you're kinda right. 9th did have a run of producer collab albums when no one was really doing that to extent they are now. Al obviously has done way more and with much bigger names, but 9th had all those multiple collab projects with Murs, Buckshot, Jean Grae, Skyzoo, etc.
He had one with Ness that never came out officially that I always thought was fire.
What other producer's underground in the 2000's were doing multiple full projects for people?

Common and Kanye. Saigon and Just Blaze. I can't think of many others. Not a of one producer one artist full projects back then like there are now.