On micro-coded CPUs Assembly is indeed an abstraction, because that is not what the CPU is running, hence why many mainframe manuals refer to Assembly as bytecode.

On x86/x64, the chips have long migrated to an internal RISC like architecture, with an Assembly => micro-op translation step on the decoding unit.

> Is assembly an abstraction ?

Yes, assembly is an (mostly fairly thin, on non-microcoded hardware) abstraction over machine code.

Assembly is an abstraction, as the others have mentioned, but many compiler people are aware of this as it affects performance.