"A Mac won't run ARM User Code."
Correction. An x86 based Mac won't run ARM user code.
However Macs (not just in labs, but in people's homes) have already used 3 very different CPU architectures. Another CPU architecture (especially a modern one such as x86-64 or arm64) would not be any problem at all. Especially given that the A9X is many times faster than the processor in several of my Macs and MacBooks that can run some version of OS X.