Roasted potatoes: should they be dry

In my opinion, it's a matter of how much time you have and what temperature your oven is set on already. I make these a lot, with a variety of dishes, so I've used multiple cooking methods.

Ideal: They roast best, I think, being boiled just for a few minutes then roasted at 475 F for 30 minutes.

Oven's already on low: If I have a more slow-cooking item already in the oven at 350 F, then I'll go ahead and put the potatoes in raw and leave them to roast for 60-90 minutes.

Need them quickly: When in a time crunch, I've also had success boiling them until softened completely then broiling until browned (maybe 5-10 minutes).

I suspect you may have created a poor-man's slow-cooking environment in there. You had meat, and liquid, and a median temperature of around 200° F, and you probably also got the bird close to "done" during the first broil, before you even left the house. This is obviously easier to do when the meat is covered (was it in a covered roasting pan?) due to the steam, but the oven does provide some insulation to begin with.

Technically when slow-cooking you should theoretically be able to speed up the process by quickly bringing the meat up to just below doneness/moisture-loss temperature (130° F) and then switching to a moisture-preserving slow-cook method like braising. I think that's what you accidentally did, but it's hard to say for because nobody was there for an hour and it sounds like you didn't check the temperature before the second round in the oven.

My guess is that the second roast at 400° F was probably unnecessary, and that the bird was already done, having been cooked in a very slow roast.

I'm pretty sure it wasn't the basting that helped. It seems to be regarded as a myth these days that basting keeps the meat moist, because the baste really doesn't penetrate the skin (and it's not the skin you're worried about). Basting is done to add flavour, not preserve moisture.

The recipe itself also doesn't strike me as anything special in terms of keeping the bird moist, aside from having a relatively short cooking time (as with any grilling/broiling) and letting the meat rest afterward, neither of which really apply in your case. It was probably the slow heat that did it.