Off the top of my head head Louise, Rachel Ballinger and Jim Chapman all have books in the works but I'd be willing to bet there are at least half a dozen other youtubers I don't know about that will also soon be joining that ever growing portion of the youtube community who can claim they are published authors. Youtube books really are a dime a dozen.Are any other YouTube's writing books? If not, then I can see why she would've thought the joke was targeted at her. I would be pretty mad too if I worked hard on writing a book and someone accused me of just using a ghostwriter.
They are so common I really can't help but wonder if she has another reason to think he was shading her personally (like it's known she's not writing her own book in the youtube community).
As for her right to be mad Emma (like anybody) is allowed to be offended be generic jokes (and state she's offended like she did at the time) but her now making it out to be a personal attack is bizarre to me because nothing about it suggested he was singling her out (again unless she knows something we don't).
(I mean to be real she probably is just that self-involved that she's making it all about her when it clearly wasn't and there is no conspiracy here I'm just having fun speculating.)