Every time that this argument comes up, people don't get their computer science right. The problem is not whether something has modules. It is the degree of coupling amongst and cohesion within said modules that is the metric.
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.