Chicken – what I’m doing called

Well, on some level, this is exactly what sous vide cooking aims to do--very gently bring the food up to the target temperature and no further. In theory you wind up with something that's more or less perfectly cooked all the way through with no part overcooked or undercooked. But of course this is typically done in vacuum bags in a water bath, which is very different from doing it in an oven, because the bag keeps all the natural juices in contact with the meat, preventing drying on the outside.

When doing this in the oven, it's unavoidable that some of the outside will get dry. Even at temperatures lower than boiling, some evaporation will occur and the longer you cook (a side effect of low temperatures) the more drying you can get. Beyond that, about the only downsides are that the fat renders very differently so the skin doesn't get crisped, and there's little carry-over cooking that goes on after the bird is removed from the oven.

I think it's definitely a matter for debate whether this kind of method results in better taste. By not getting things hot enough for the Maillard reaction (browning, basically) I think you're leaving a lot of flavor potential untapped. However, it's undeniable that, ignoring possible air-drying that happens, you'll have little to no risk of overcooking. That alone will increase your chances of a good result because overcooking has to be the most common mistake people make.

As to why you got your 140 degree reading when the meat near the bone was still pink, I think there are two things at work. One is that I understand that the meat near the bone is just naturally more pink and that it's fine to eat in that state if the temperature is right--there's some science behind it but I just can't find the article right now. The other thing is that there's always some variability to sticking a thermometer into a roast, and you might not have gotten your probe into the least-done part.

If you are wondering explicitly about Nandos, it is marinated for 24 hours before its cooked. It is then cooked before its ordered:

[The chickens] are cooked in a special oven for an average cycle of 30 minutes, reaching a temperature of at least 75C, then kept in a warming drawer at a temperature of 63C.

Source

When an order is placed, they take the Chicken and finish it on the grill.