I would interpret it like this: "at first glance people didn't think he was a gifted pianist, because he seemed too beautiful, and his manner of speaking was too laconic" (presumably he wasn't boastful). Am I right?
Thanks in advance!
Update: it seems that my interpretation is right but the author could select more appropiate words for this idea. Yes, perhaps it's an example of bad prose (maybe not very typical for the book, I will comment on it more extensively in comments below). Still, it's a memoir, and I think the author just tries to convey her first impression of the pianist. Also there are some types of physical beauty that make one unwittingly look frivolous or young for one's age.