Aside: if Apple's M1 processor is ARM-based, why do we bother making the distinction? If anything I feel like this hurts more than it helps because Windows and Linux need better ARM support too.
Because, quite frankly, ARM silicon is held back by other vendors. Apple is unique in the ARM space in that they actually make an attempt at retaining compatibility across SoC iterations. Linus Torvalds has famously gone on multiple rants about how much of a mess ARM chips are - every new chip usually meaning a full reshuffle of the register map and new drivers everywhere. The work being done on Asahi Linux, on the other hand, is very likely to carry forward to M2, M2 Pro/Max, etc.
FWIW, M1 Macs are also one of the few ARM platforms that will let you[0] actually install regular GNU-and-Wayland desktop Linux. I think the current crop of Windows on ARM machines will also let you install Linux, of course. But most Android hardware ranges from mildly to very hostile to users who want regular Linux - even if there's a bootloader unlock, the hardware is going to require all sorts of extra vendor-proprietary drivers that won't work with a standard distro. This also ties in with what I said before about backwards-compatibility across multiple SoCs.
The reason why people treat the M1 as separate from all other ARM hardware is because it actually acts like a computer; providing the performance and control you expect from something bearing the name. Most other ARM vendors are not aiming to provide that.
[0] You do have to sign the kernel with your Owner key, but that's manufacturer-supported, so it counts.
I guess because M1 describes the entire platform in shorthand (CPU+GPU+performance characteristics), whereas ARM only describes the CPU instruction set.
According to marcan’s Twitter, Apple actually broke the ARM spec in ways most companies are literally not allowed to do. Part of the deal is likely that Apple was not allowed to call their chips ARM.
FWIW, M1 Macs are also one of the few ARM platforms that will let you[0] actually install regular GNU-and-Wayland desktop Linux. I think the current crop of Windows on ARM machines will also let you install Linux, of course. But most Android hardware ranges from mildly to very hostile to users who want regular Linux - even if there's a bootloader unlock, the hardware is going to require all sorts of extra vendor-proprietary drivers that won't work with a standard distro. This also ties in with what I said before about backwards-compatibility across multiple SoCs.
The reason why people treat the M1 as separate from all other ARM hardware is because it actually acts like a computer; providing the performance and control you expect from something bearing the name. Most other ARM vendors are not aiming to provide that.
[0] You do have to sign the kernel with your Owner key, but that's manufacturer-supported, so it counts.