I assume the author of the library works under a hard real-time constraint. Under such circumstances (an example would be low latency audio) you can not tolerate the latency impact of a sporadic syscall.
It's not about performance so much as determinism.
Generally lock-free is better for these situations as odds are when you hold a lock at least some CPU cycles are used for something that isn't directly modifying the data that needs the lock - those cycles the other CPU can touch it when lock-free. (note that when using a lock there is a trade-off, often it is better to hold the lock for longer than needed instead of dropping, doing a couple operations and then locking again)
(All wait free algorithms are lock free because necessarily if every thread is guaranteed to eventually make progress then overall progress is definitely made)