Who said? K&R is pre ANSI C, so pre any standardization.
The concept of the C standard library post-dates ANSI C, so to say that they "changed" the language in that they more closely documented the behavior of the language and the standard library means that, for all intents and purposes, the standard library is part of the language.
Which is exactly the point you are making about FORTRAN and PL/1. That they have standardized functions that perform in a documented manner and so can therefore be optimized by the compiler.
That C compilers understand the defined behavior of strcpy et al means that they are allowed to optimize the implementation.
