I think there would be some overlap, but if the site were to launch I see the writing oriented questions focusing more on marketing, typography and formatting, various publishing platforms (e.g. Leanpub) and possibly distribution related issues. From the proposal, it's more about the logistics of writing than writing itself - they seem to assume that you're capable of putting something interesting in the book.
Ever since the closing of our gadget related site, there really hasn't been a home for questions about simply using an ebook reader, so I can see why it's getting some traction on that end.
If it manages to gain critical mass, sure - I'd definitely consider it a sort of sister site. It seems like it picks up where Writers SE leaves off. You bled, you cried, you sweated; now you have something to publish and need more help.
I'm .. uncertain that the topic will be sustainable, but do feel free to commit to the private beta provided that you've got some real world questions to ask or expertise to share :) Still, I'm not certain many parts of that scope would be off topic here, it seems like there just hasn't been much interest in it.