It doesn't need cgroups or containers, because every process has its own namespace, its own view of the network-global filesystem, so everything is in a container by default.

It doesn't need a microkernel, because the concept of microkernels is to split a big monolithic kernel into lots of small simple "servers" running in user space, and have them communicate by passing messages over a defined communications protocol, some kind of RPC type thing. It works, and QNX is the existence proof. But it's really hard and it's really inefficient -- of which, the HURD and Minix 3 are the existence proofs.

So most of the actual working "microkernel" OSes kludge it by embedding a huge in-kernel "Unix server" that negates the entire microkernel concept but delivers compatibility and performance. Apple macOS and iOS are the existence proof here. (It could be argued that Windows NT 4 and later are also examples.)

Plan 9 achieves the same result, without the difficulties, by default by having most things user space processes and communicating via the filesystem.

Disclaimer: this is my very rudimentary understanding. I am not an expert on Plan 9 by any means.