Implement a function that determines whether a string that contains only letters...

Clearly the above mentioned points contradict this point. Also description is shared amongst all languages, so changing the requirements of a specific language is not feasible, let alone the amount of languages and solves as of now ...

Na, it's important to give that kind of feedback. We see far too often solutions with bad time complexities that are voted as best practices while there are more efficient and yet simple solutions.

The user is 7kyu. That they solved the problem correctly with such readable code is impressive. Give them a break :)

I know that, thank you) Obviously the solution wasn't designed to handle NULL pointers and chars other than latin letters, since it's unnecessary for the kata

You're correct, and it's not unusual in C for functions that take a null-terminated string to behave that way. Try it yourself, most implementations of strlen() or printf() will segfault if you pass NULL.