Systemd is an ecosystem.
"Ecosystem" == vendor-managed closed system.
True interoperability does not produce "an ecosystem", it produces standards with (at least potentially) multiple implementations.
It contains a number of different binaries. They may be part of the same codebase, but so what?
So it's contrary to the idea that modularity is achieved through clearly defined interfaces, so "the same codebase" is an unnecessary luxury for lazy designers.