I mean all of open source software (as we traditionally defined it) relies on an ISA, which is actually executed by microcode that isn’t available to the public?

I don’t see how this is any different. It’s like saying Intel has to publish their transistor physical library and microcode to be “truly” open source. Like I don’t have the internal notes about the most recent microcode changes (which is akin to “training process” in the analogy).