You can have a well-integrated system without having tight coupling between components. Interfaces are a thing, and a high level of integration just means having a good collection of interfaces between well-defined conceptual components with well-defined capabilities. The actual software that implements those components is entirely separate.
I don't know what you are arguing. Replacing a component in a tightly integrated system means that the component has to be compatible to that tightly integrated system, meaning lots of assumptions and replication of functionality.
This means components have to tightly conform to the components they are replacing, which obviously decrease modularity as these components are harder to maintain for available system configurations.
This isn't about whether someone can rewrite a part of systemd, but whether you can freely mix and match, which tight integration works against. Obviously
