"they need other things" - like the extremely strong app eco-system of Android (in comparison to Windows Phone, specifically). WP is a strong OS with some great design choices etc (IMO, obviously); it's main downfall from my point of view is the lack of apps. Forking Android (not necessarily just "skinning" but a step beyond that) is an obvious solution to that issue.
The entire article is all about how it's not really an obvious solution at all. Amazon is forking Android, and they have rewritten many of the GMS APIs, and yet, many of the apps do not run on Kindle Fire like they do on Android.
