They're both RISC, but P9/P10/etc is based on IBM's own Power microarchitecture (possibly open sourced / open standard), I wasn't aware it had anything to do with Arm? I could be hopelessly wrong...
There are other interesting ISAs that are not arm64. Sadly, the only ones that still could be viable desktop (more like deskside) are either server-grade ARM or IBM's POWER (and none of them makes Mac Minis).
They're both RISC, but P9/P10/etc is based on IBM's own Power microarchitecture (possibly open sourced / open standard), I wasn't aware it had anything to do with Arm? I could be hopelessly wrong...
Or do you mean something different with "POWER"?