I find it hard to believe that Bix, home recovering from an illness and contacted by a local reporter, would have had easy access to these sources. It's much more likely that, faced with a subject with which the reporter had little expertise, he or she would go to the newspaper's morgue or wire service and do some research on background; that's what any half-decent writer would do.
It also seems improbable that Bix had such a photographic memory that he could have repeated what he had read so close to verbatim in a face-to-face interview. If he'd had that kind of memory, (which he claimed he didn't have in a letter from Lake Forest) he could've aced all his high school exams! It also seems unlikely that Bix, not a prolific letter writer, would have sat down and copied out all that material to send to the reporter, even if he had had copies before him. However, on the other hand, I don't think this interview would have had a tight deadline, making the written interview at least a remote possibility to be considered.
But here's another (admittedly half-baked) theory. Could the reporter have actually shown Bix these clippings from the morgue and asked for his take on what was said about jazz in them? (That would be a reporterly thing to do.) Might he just have laughed and said something like, "What the heck? I can't argue with the experts," perhaps making the reporter feel marginally justified in "borrowing" the quotes and attributing them to him? (It would be informative to track down examples of this reporter's work to see if this theory makes any sense.)
Thanks, Brendan, for sharing this very interesting information. I hope your book research continues to uncover some new takes on these "mysteries."