To reiterate, claiming that C can be described as "portable assembly" is not a claim that it is literally a package of assembler macros that emit deterministic machine code for each individual source expression.
I linked these in another comment, but here's some examples of straightforward-looking integer addition emitting more complex compiler output for other languages that compile to native code:
Haskell: https://godbolt.org/z/vdeMKMETT