You're almost there. Yes, there's a narrower definition. Yes, that means that crashes are outwith the kernel. But no that only means that the circle around what bit is named "the kernel" is smaller. Importantly, it does not mean that it is impossible to "crash the whole operating system".
Microkernels aren't a magic bullet against crashing an entire operating system, because a crash in a non-kernel component that the entire operating system relies upon, such as a central shared rendezvous server, or a server that handles "magic" values, or a central local security subsystem server, or a shared fundamental "personality subsystem" server, still crashes the entire operating system, no matter that the crash isn't in the part of the operating system that's labelled "the kernel".
I think now we need to define what's the "whole OS". From a user's perspective: the whole OS crashed (became unresponsive). But from a developer's perspective the OS worked, as it was still logging, responding to network, etc.
As I see it a micro kernel has simply a narrower definition of what should be in "kernel land" and what should be in "user land".