I think we need a separate term for "internal modularity" (Linux, Systemd) and "external modularity" (Unix, the Internet).

The former depends on internal interfaces, where individual modules help separating concerns but can't be swapped out or reused in other contexts, whereas the latter relies on formal interfaces, allow components to be swapped, and promote reusability. (Also, the former can be monolithic or not, but the latter can never be monolithic.)

"Unixy" is sometimes used, but it's not a great term.