Chicken – Sous vide a whole chicken : temp/time

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.

You cooked it at too low a temperature.

Sous vide is intended for meat where you want the protein to remain tender. It shouldn't have any sinews. Think chicken breasts, or the long filet along the spine of a pig. This meat gets nicely cooked at 60-65°C (depends on the animal), and tough and dry above that.

Meat marbled with sinews has to be cooked at a temperature where the sinews (collagen) melt into gelatin. This happens at about 70°C at least, and takes hours. Since the muscle fibres are already toughened at that temperature, there is no reason to hold it low; you can put it at full boil in a normal pot and cook it there, you just have to wait long enough. In theory, you could do it in a sous vide bath too, but you won't get any of the benefits sous vide gives to tender meat.