Rather than the instruction set, I assume he's referring to the lack of a standardised boot process, which wasn't addressed until recently. Lots of single board computers out there that don't have upstream Linux support due to badly made firmware, device trees, and other weirdness.
Although I agree that this is the likely reason, it is a bit outdated. ARM made a standardized boot requirements document last year, it's now up to the SoCs to implement it.
Yeah it's why I say "which wasn't addressed until recently"
Sorry, I didn't see it
It's almost like his religious convictions mean he's never heard of this 'M1' thing.
Realistically, I think he's saying "Linux on anything other than x86 is a dumpster fire", which is fair.