My understanding is the use of Open was used before this by Symbolics and possibly other lisp machine manufactuers. I'd argue too that their systems were more open, where the user could read the system software and patch it in while it was running.

The foundations of the open source movement too stem from these communities (the GPL) not from NeXt. It would appear to me, like other Unix manufacturers, NeXt claimed openness when mostly it was marketing-speak for "based on Unix".