There is significant variety between distributions regarding which systemd project modules are used by default. Many distributions also allow systemd modules used by default to be easily replaced with non-systemd alternatives. This speaks to the level of coupling and cohesion between systemd modules being low enough that they don't form a monolith.

Waving around the terms without understanding them, as you are doing, demonstrates exactly what I mentioned. Someone who actually knew the computer science would not be talking of low cohesion here, or of modules forming a monolith.