I don't know if it's as much "obscure concept" as "failing to adapt to the new system by reimplementing your old system in it". E.g. you could technically run a container by putting everything including code in a mounted volume and set your CMD to call it. That would be awful, you get all the downsides of containers with none of the benefits, etc, but if you refuse to use the new tools 'with the grain' that's the kind of thing you end up with.

In the context of Silverblue I think they have a reasonable use case for pet containers. If your base system and all your containers are immutable that leaves you nowhere to do learning/testing/experimentation.

In this context it seems to be something else