The POSIX standard defines ed as one of the editors that must be present for a system to be standard complaint, the other being vi I believe. You can always expect ed to be available to you on a Unix system.
It is true that all certified Unix systems follows POSIX, but it doesn't mean that non-certified systems are forbidden to follow POSIX. Most Linux distributions have ways to turn to 98% compliant, and BSDs have always strive to follow POSIX.
Two of those programs predate POSIX and do not indicate that OpenBSD follows POSIX.
They are making their operating system to the design they like best. Some of it happens to coincide with what POSIX asks for, but it is merely coincidental.
I vaguely recall some interview with or blog post / talk by an OpenBSD developer, where they said that POSIX tended towards adopting whatever GNU/Linux did, and that the BSD crowd's interests were not taken into consideration that much.
I don't know much about the process behind POSIX/SUS, but I can understand how OpenBSD developers wouldn't be super enthusiastic about POSIX compliance if they felt their input was falling on deaf ears.
FWIW, I did use the SUS as my main reference when writing a few hobby projects, and OpenBSD gave me no problems whatsoever. macOS, on the other hand, which is a certified Unix, did not support barriers at the time (that was ~10 years ago, I have no idea if Apple added support since). (I know barriers are optional, but come on.)
I wonder what benefit the certification has these days. IIUC, it used to be a requirement for contracts from large corporations and government agencies, but these days the number of commercial Unix systems is fairly small, and RHEL is so common in those environments, I wonder if it there's still places where it's required.
