I don't see any problem with saying that Android is not an open system. They use open source, but the way it is distributed takes away many of the freedoms of OSS. Also, most of the system is really in the Apps, many of which are closed source. In other words, Android is as much open source as the iOS is, because it is based on an open source kernel but the user space is full of non-open components. And good luck trying to contribute something back to the owners of the system...

I think you're right, and that sideloading (which is really just normal loading, as people have done since the dawn of computing) is the most important test of openness for a platform. If you can write a program, and hand it to me somehow, and I can install it - and the platform developer doesn't have a veto over it - that means the platform is open.

Platform openness is really important, too. It means that people can write and share applications that the platform developer may not approve of - benchmarks, secure network code, games with controversial content - the list goes on.

That isn't to claim that an open source platform isn't a good thing. But open closed-source platforms have been a huge boon to the world since the dawn of computing, while closed closed-source platforms are problematic.