Using an API in a language says nothing definite about how that API is implemented.
The API could well be implemented in what you call "pure C".
So that term isn't very meaningful in the way you claim it is, and there is no need for you to be policing it. If it was about "dependency free" could maybe agree more.
The term is often used to indicate that the source code of the project itself does't make significant use of other languages like C++, and to allude to a sense of simplicity (which is often subjective).
This "pure c" won't compile with any random C compiler, unless it happens to be a C compiler in a POSIX environment.