I think you missed the point of the article completely.
Which is:
Containers are not actually a _thing_. BSD Jail is a _thing_, Zones are a _thing_... Linux containers are just a particular configuration of _multiple things_.
PIDs in containers CAN be like PIDs in BSD Jails... if that is what you want. It's up to you to use what Linux primitives you want in your containers.
For example:
I can run a application in a 'linux container' that shares PID, user, and network namespace with the main OS.. and the only thing that is different is that the file system is namespaced. I can run cgroups without running namespaces. I can run namespaces without cgroups.
Now if you want to talk about _Docker containers_ then, yes, that is a _thing_, but it's just one of many different possible ways to have Linux containers.
No, from the article 'Solaris Zones, BSD Jails, and VMs are first class concepts.' It's just happens that jail as a name, jail as a cmd tool and jail as a system call bears the same name. Nothing stops one to implement superfancyjails on top of that system call. Same story with Linux containers, we have clone(), unshare(), and setns() and couple of popular implementations on top of them.
Thus, lets say, 'man systemd-nspawn' container is _thing_ as 'man 8 jail' is _thing_
You're splitting hairs to explain something that doesn't matter. The article stands well on its own two feet without nitpicking the similarities as you have done.
Which is:
Containers are not actually a _thing_. BSD Jail is a _thing_, Zones are a _thing_... Linux containers are just a particular configuration of _multiple things_.
PIDs in containers CAN be like PIDs in BSD Jails... if that is what you want. It's up to you to use what Linux primitives you want in your containers.
For example:
I can run a application in a 'linux container' that shares PID, user, and network namespace with the main OS.. and the only thing that is different is that the file system is namespaced. I can run cgroups without running namespaces. I can run namespaces without cgroups.
Now if you want to talk about _Docker containers_ then, yes, that is a _thing_, but it's just one of many different possible ways to have Linux containers.