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.