Of course containers are complicated! The kernel's documentation for cgroups is 13 separate documents! You can't even "download" a container without a complicated tool to "manage" it on the disk! They are quite complicated.

Also, cgroups aren't containers. "Containers" is a loosely defined concept encompassing Linux's common implementations of cgroups and namespaces and chroot environments and networking and union filesystems. Complicated x5.

Also also, that eli5 assumes a lot of Linux knowledge most five year olds don't have.