So what? He likes Scala. Likes it enough to write a book in his spare time, of which he has precious little since he's, oh yeah, a full-time developer at one of the biggest, fastest growing sites on the face of God's green earth.

So it's a (mild) conflict-of-interest (really, it's just a promotional opportunity). Doesn't make the arguments any less valid or his book any less worth picking up. Nice thing about code is that it's like science - every claim can be tested ad infinitum.