I didn't say anything about ARM specifically either. If I were a random tinkerer, I would choose based on these factors:
* Ecosystem support (libraries, stack overflow, docs)
* Quality of tools (compilers, debuggers)
* Price/availability
ARM is probably a good choice based on these factors, due to things like Raspberri Pi and Arduino (not all ARM but maybe the more popular ones are?). Open source hardware architecture doesn't really factor in anywhere here.