As best I can tell, tmux is developed primarily by Nicholas Marriott for OpenBSD and made portable with libevent. The original paper uses OpenBSD, and here[1] he says: “I preferred to work on it in base: I felt tmux would be improved by being part of OpenBSD.”
The author developed tmux on OpenBSD because that is what he used, but it was never part of the OpenBSD Project. It was a port prior to being included in the base install. This is no different that how FreeBSD includes software in its base install, yet this doesn't make the code part of the FreeBSD Project.
The tmux GitHub repo is the reference implementation. As an example, OpenSSH is developed internally to OpenBSD and the project creates a separate portable version. tmux is the opposite, it is developed independently of OpenBSD and the project maintains its own implementation.
The tmux GitHub repo is the reference implementation. As an example, OpenSSH is developed internally to OpenBSD and the project creates a separate portable version. tmux is the opposite, it is developed independently of OpenBSD and the project maintains its own implementation.