Real open source systems have outside contributors and stakeholders, and developer communities. Android mostly just has app developers and a "hacking" scene (xda etc).
Yes there's a small band of Quixotic guys trying to make an open source fork limp along but it's comparable in size (and odds of success) to Amiga users/developers. It's an unusable tech experiment that only runs on a couple of antiquated phones and has no services, not the Android that people know.
The only openness-related reason to prefer Android over iOS is that they still let you install what apps you like on it (sideloading) and the app store policies are slightly less draconian than Apple's.
http://redmine.replicant.us/projects/replicant/wiki/Replican...
Plus there's absolutely no way to upstream their work, so it's a massive maintenance burden (one of the reasons they're still on 4.2)
AOSP is just what zurn said above - a code drop. If Android was really an open platform, Replicant either wouldn't be needed or Replicant devs would have much less work to do. Openness of Android is great for Chinese vendors making cheap tablets, so they have a ready system to put on them for almost free - but it's not so good as an open platform.
If you aren't going to be part of the virtuous cycle of the Android ecosystem / OHA in general and license GMS, it makes less sense to use it as a base for something as grand as whats described in the article.
I think this is the way Google designed it.
What makes little sense is to start again, mostly from scratch (or from MeeGo or its successor, which is basically from scratch). This project is a failure before they started working on it.
“the agreement places a company-wide ban on Android forks, saying OEMs are forbidden from taking ‘any actions that may cause or result in the fragmentation of Android’ and specifically disallows distributing or encouraging a third-party to distribute ‘a software development kit derived from Android.'”
Not so very open after all?
The people who have forked android have great difficulty finding OEMs to manufacture thrir devices as the list of those who don't have agreements with Google is rather short.