I believe they mean that the window of time in which it's delicious is narrow.

(Because a rack of lamb is fairly thin compared to many meats we roast)

To which I would say: meat thermometers are an amazing gift.

Yeah, the window of time when you need to pull it so that it's good. As in, pull it 2 minutes one way and it's not done, 2 minutes the other way and it's overdone.