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.
I make stock from my leftover chicken bones and vegetable scraps. I just dump them in the pot still frozen, cover with cold water, and heat until it's a bare simmer. Simmer the stock for 2-3 hours or so, or until it tastes like soup instead of water. (But be careful not to boil, since the stock can get cloudy if you let it get up to a full 100C.) I have made this with both cooked meat and uncooked.
Uncooked chicken (whether frozen or not) will give you a mildly-flavored white chicken stock. If you brown the meat first, you'll get a stronger flavor and a brown chicken stock. (See here for a discussion.) If your chicken isn't cooked, you'll want to roast it to get all the nice brown bits from the Maillard reaction, which will defrost the chicken in the process.
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The more exposure to air it has the dryer it will become over time, so keep it in some sort of sealed container or wrap it in plastic-wrap (or foil) immediately after purchasing to lock in the moisture.