That is the myth, in practice ISO C specifies an abstract machine and libc is impossible to be written 100% in ISO C, some functions require either external Assembly or language extensions for their implementation.
Check language support across Small-C, Turbo C, Quick C, Aztech C, Metrowerks C and many others from those days.
Likewise it was possible to do the same across Pascal compilers that implemented variants of ISO Extended Pascal/UCSD Pascal, Modula-2 and a couple of other high level languages.
The whole point is that contrary to its sales prospect, not only does C also require to write Assembly, it doesn't have any language support for modern hardware CPU/GPUs, just like any other high level language, requiring extensions to the standard, not necessarly portable across implementations.