> […] on ARM64 […]

Well, if you target a specific architecture, then of course you can assume more guarantees than in general, portable code. And in general, a processor might distinguish between non-atomic and relaxed-atomic reads and writes – in theory.

But more important, and relevant in practice, is the behavior of the compiler. C, C++, and Rust compilers are allowed to assume that non-atomic reads aren't influenced by concurrent writes, so the compiler is allowed to split non-atomic reads into smaller reads (unlikely) or even optimize the reads away if it can prove that the memory location isn't written to by the local thread (more likely).