tko

I think this is just a terminology difference. The documentation says that "Add Ons" are not supported in Container and Core, but "Add Ons" means the easy button you press to install those services. All of those Add On services are just containers that HAOS manages for you. Every single one of them can be set up as a container manually and function the same as the official "Add Ons."

I don't know for sure, but I wonder if the reason for this is that it's not technically possible for a container to manage other external containers. Does anybody know about this?

To be fair, Addons are just other containers. If you're using a Docker install for Home Assistant, I think the idea is you already have a handle on your docker host, and you're capable of adding whatever other containers you might need.