The core functionality is and must be built in. You can't do very much with it in that state, so Multi-Account Containers is an user interface to configure and access most of the user-facing functionality. Some of that can be enabled just by toggling a couple of about:config settings, though. And as far as I know, though, it doesn't have privileged access to container APIs, so you could replace it with other addons.
So whether it's more correct to be referring to the built-in functionality (including some of the UI elements), or the add-on, depends on exactly what you're talking about. And it's hard to distinguish.
I see, thanks for the clarification. I wonder why Mozilla didn't just ship the add-on. It seems far-fetched to think it was to make the add-on replaceable by another.
So whether it's more correct to be referring to the built-in functionality (including some of the UI elements), or the add-on, depends on exactly what you're talking about. And it's hard to distinguish.