> Wasn’t the “open” at the time meaning “open system” as a system that is open for external connections (aka networking) and not so much open as in “open source”?

Networking was the initial impetus, but the phrase came to include programming interfaces, which is why POSIX was considered such a big deal. The idea was to promote interoperability and portability, as oposed to manufacturer-specific islands like those from IBM and DEC.