> If it would the majority of optimizations compilers do today would not be standard conform.
They aren't.
So you can easily tell, just by looking to the C source code, if plain Assembly instructions are being used from four books of ISA manual, if the compiler is able to automatically vectorize a code region including which flavour of vector instructions, or completely replace specific math code patterns for a single opcode.
not rally, or at least not in a way which would count as "high level assembler". If it would the majority of optimizations compilers do today would not be standard conform.
Like there is a mapping to behavior but not a mapping to assembly.
Which is where the abstract C machine as a hypothetical machine formed from the rules of the standard comes in. Kinda as a mind model which runs the behavior mappings instead of running any specific assembly. But then it not being ambiguous and complete doesn't change anything about C not being high level assembly, actually it makes C even less high level assembly.