It's been several years since I read it, so I'm fuzzy on the exact characterizations, but the notion the author seems to lean into is that most bassists are (as you say) nerds who aren't interested in the spotlight, but they provide a fundamental bridge between melody and rhythm that enhances the other performers without necessarily standing out on its own. Meanwhile, the "drummer" character in this book is less Ringo Star and much more Jon Bonham or Neil Peart. He's got, if not an active desire, at least no aversion to the spotlight.

Of course exceptions to every stereotype exist, and there's a very valid argument to be made that a rock drummer ought to be the archetypical support class, but the division of stereotypes made sense to me while reading.