There is a good practical question and interesting issues raised here, but this part:
"my understanding of OSH is that all its components must be open source."
I would say your understanding is wrong, where did you get that from?
The design (i.e. how to arrange components) must be open source. Ideally the components would be Open Source too, but practicality dictates that components must be bought "off the shelf". IOW, overall the design should aim for "as Open Source as much as possible".
The thing about hardware is that to realize a design components must be bought, and those components often contain proprietary IP. Of course, the same problem applies to Open Source Software, to run it I need to buy a CPU from Intel, AMD, etc.
So while all open source designs must rely on some proprietary IP as a base platform, the practical question is how easily can the design be moved to another platform (portability), and how easily a component can be substituted (fungibility).
There is only one source of AVR chips and that is Atmel. Other components, like a 3.3V regulator can be easily substituted with other.
The Arduino core software does not rely solely on AVR architecture, in principle it is portable to any CPU platform, in practice any platform that is targeted by GCC. The fact that some Arduinos are not AVR e.g. Due, shows that.