The output side of strncpy() might not be a str___() function, but AFAICS the input side of strncpy() is clearly a str___() function, since it stops reading (but not writing) at the first NUL byte.

But it's not an str* function, it's an strn* function. And most (though not all, that would be too easy) work on fixed-size (hence the n) nul-padded strings.

No, it isn't a string function of any kind. "A string is a contiguous sequence of characters terminated by and including the first null character." § 7.1.1.

Calling a bespoke byte-sequence data structure a "string" is inaccurate. Treating strncpy() as a string function is erroneous and can easily lead to memory corruption.