Indeed it is mentioned. I wrote the comment because repeating the check in the kernel on a user provided value is what distinguishes futexes from a condvar. Of course muted+condvar+int is not a replacement if you're implementing pthreads, but it's also much less efficient than futexes if you're implementing a synchronization primitive not provided by pthreads.

(The most common use of futexes that I have seen outside glibc is to implement binary semaphores. Binary semaphores can be a lighter-weight alternative to condition variables when only a single thread can ever wait on an event).