As far as I understand, x86 is a duopoly, and AMD is allowed to produce it because antitrust litigation resulted in Intel being required to allow a second party

As far as I understand, x86 is a duopoly, and AMD is allowed to produce it because antitrust litigation resulted in Intel being required to allow a second party to implement their instruction set. Also Intel is a very different entity than Apple. They produce commodity hardware, where Apple creates consumer experiences. Apple's incentive is to keep M1 closed to give their software ecosystem an edge vs. competitors.