They took it from the NCAA official record book. All of those polls are listed as Major Selectors - they're listed out on page 112 under a big banner that says
NATIONAL CHAMPION MAJOR SELECTIONS (1896 TO PRESENT)
And yes, it lists a bunch of dumb polls and computer systems that I agree shouldn't be included. But that's the list they're taking from - the final national poll leaders according to those selectors.
There's an additional list on page 125 which cuts through a bunch of that chaff to specify consensus national champions, and that's where they get the list of national champions from. I'm absolutely not saying I like the list, I'm just saying it isn't the PAC12 making it up out of nowhere. They're pulling from the official record book.