rkt is not supposed to require systemd, but it is interesting that you asked that, as I had to check that earlier due to a warning (that looked like an error) when looking for non-existent libsystemd.
CoreOS (the origin of rkt) definitely uses systemd, and the person who expressed the surprise at cgroup not being mounted was (I think) a coreos person, so they might be used to that approach.
No clue why such a thing would end up as mounted, but systemd is totally foreign to me!
I think my takeaway here is that there is no reason for cgroup to not work properly. I thought the "read-only file system" part of "Failed to create root cgroup hierarchy: Read-only file system" might be related to a tinycore thing somehow, but perhaps not. I'll chase it on the rkt side.
Thanks!