The "at least" is meant to convey that the API doesn't promise only one will be woken. It's very similar to the way that pthread_cond_signal() is documented by

The "at least" is meant to convey that the API doesn't promise only one will be woken. It's very similar to the way that pthread_cond_signal() is documented by POSIX to wake at least one thread waiting on the condition variable, and for the same underlying reason.