Maybe I'm misreading, but considering it OK to leak memory contents across a process boundary because it's within a cgroup sounds wild.

It wasn't any cgroup. If you put two untrusting processes in a memory cgroup, there is a lot that can go wrong.

If you don't like the idea of memory cgroups as a security domain, you could tighten it to be a process. But kernel developers have been opposed to tracking pages on a per address space basis for a long time. On the other hand memory cgroup tracking happens by construction.