It does seem weird to protect user space memory from the kernel but not the memory of kernel subsystems from other kernel subsystems.
But at that point we're at a microkernel design that needs context switches between kernel processes... Good, I'd welcome that. This feature just implements a level of security that the whole kernel needs to catch up to before it makes sense.
E.g. if syscall S reads kernel memory without properly checking pointers/offsets (maybe GCC optimized them out), then process A can possibly use that to steal process B's secrets. Or maybe the kernel does check the pointers/offsets, but isn't hardened against yet another speculative execution side channel attack your processor is vulnerable to, so leaks some information via timing information.
MAP_EXCLUSIVE breaks that exploit, and to unbreak it you need both syscall S and another buggy syscall (or maybe another row hammer attack) - to write kernel memory - and to successfully leverage that into tricking the kernel into remapping process B's pages.
No panacea, but I could see it rasing the bar in exploit difficulty as part of a defense in depth strategy.