Wizardly workarounds for broken APIs persist long after those APIs are fixed. People still avoid things like flock(2) because at one time NFS didn't handle file locking well. CLOCK_MONOTONIC_RAW is fine these days with the vDSO.
It would be weird, even for AIX, to support POSIX byte range locks and not the much simpler flock.
I’m using Linux rather than AIX.
fcntl(2) locks are supported (as long as they aren't OFD), but flock(2) locks don't work across nodes.