Huh? I don't think anyone is really doubting the authenticity of the book. At least not yet. They're doubting that Harper Lee has actually authorized its release with full knowledge of what that means.
Oh, it's not the most likely hypothesis; not having access to the book yet I can't even say if it'd even be a relevant question. My most likely depressing scenario is that it's a less interesting novel that wasn't published at the time because it simply wasn't as interesting, and Harper Lee isn't in a position to evaluate it because she hasn't read it in fifty years.
I'm hoping for a happy ending here, though.