Vetting the cargo

That architecture-specific assembly language implementation which is aggressively optimized for performance on large strings rather than readability or ease of implementation was, I am sure, quite challenging to implement, but this falls into the realm of "accidental challenge arising from the chosen method of implementation" rather than "essential challenge arising from the specification".

A naïve compliant implementation of strlen() in native C, on the other hand, is in the realms of "too trivial to copyright":

size_t strlen(const char *s) {
size_t len;
for (len = 0u; s[len]; ++len) {
/* do nothing */
}
return len;
}