[jerf]

A term's utility comes from its ability to separate things into different categories. A definition of "container" that includes everything is therefore useless, because if everything a container, there is no information in the statement that something is a container.

In Bartosz's case he's probably making the precise point that we can abstract out to that point and that at a super, super high level of category theory, there isn't anything that isn't a container. However, that's a didactic point, not a general truth. In general we programmers generally do mean something by the word "container", and functors can indeed include things that are therefore not containers.

Moreover, I would say it's not what the author was thinking. The author is not operating on that super high level of category theory.