Why a warning horn if you are in manual? It's a configuration that the crew can employ if they wish. As for 10000 hrs experience, it doesn't mean a thing, only that they have not been challenged in all that time with a simple problem such as this. The reliability and automation built into aeroplanes these days means that the quality of operator and of recurrent training can be variable to put it mildly. This situation would have been a non event to a competent crew.