Well, they could have just paid attention to the show, which not only explained who everyone was, but made jokes about it. What did you think "Round of the Baskervilles" was about?
The main requirement is the ability to read an autocue. Which some of them fail. I thought he did well at that, and had the necessary aplomb. He wasn't especially witty but he did enough of his own off-the-cuff ripostes to suggest he could be good if given more time to relax into it.
As you implied early, he is a BBC rising star. Sherlock was very successful.