I think the Doc does these “vacations” on the fly. I note he said he was a little hung over at the start of the show. (or maybe a lot!)
I think that you are correct. It reminds me of when I had a government job for a few years. Pretty common there.
Again, he's earned it IMO but I really prefer a fill-in but perhaps the thinking is that new listeners should hear a recording of the real thing.