> this is an essential idea encompassing microkernels, and other “capability-based” systems

Tiny nit, but microkernels don't imply a capability based security model. For instance Mach, QNX, Redox, etc. aren't capability based.

It's a very good idea for your microkernel to be capability based because it cuts a lot of validation out of the critical path for IPC, but it's by no means a requirement.