There is no correct or wrong definition in my eyes…
The purpose of monitoring is to be able to adapt, this is what Plugins come in with Icinga 2. You should be able to monitor any system. Maybe you have to code a new check…
While we have the Icinga 2, it is not the standard way of monitoring, it just allows you to access different contexts, satellite installations for monitoring in other networks, or local access to resources like status and files.