I'm more sympathetic to the mainstream usage. People are (attempting to) use an abstraction: a "container" is an isolated process runtime environment without virtualization overhead. That abstraction seems useful to me. Ideally it would be usable without too much leakiness, in which case its users would not need to be aware of implementation details like cgroups and namespaces. In practice, all abstractions are leaky to some degree and effective use of an abstraction often eventually requires a more sophisticated understanding of the details beneath the veil of the abstraction. But that doesn't mean the abstraction is totally useless or completely a mirage or anything, it's just a leaky abstraction like all others.

If you say that a container is not a first class object but cgroups and namespaces are, I can just as easily say that cgroups and namespaces aren't first class objects, they are just terms people use to describe a combination of system calls. It's just abstractions with different amounts of leakage the whole way down.