Exactly. "Docker" is boring, everyone uses it, everyone knows it, no one really wants to rewrite it (on Linux) except for parochial infighting or religious license reasons.
But Linux containers[1] are actually fascinating stuff, really powerful, and (even for the Docker experts) poorly understood. The point of Bocker isn't "see how easy it is to rewrite Docker" it's "See how simple and powerful the container ecosystem is!".
[1] Also btrfs snapshots, which are used very cleverly in Bocker.
I like doing similar tricks with zfs snapshots for containers on linux and jails on fBSD.
(this is not intended to start a filesystem argument, I'm doing it with zfs because I already know zfs and it's available on both of the OSen I care about; if you already know btrfs and are only running things that support it, clearly you should use that for the exact same reasons ;)