From the talk linked above, they went to considerable effort to design a system with a cheap processor which nevertheless contains an mmu, and so most other embedded kernels, which assume the lack of one, are not applicable. So the point of writing in rust is that they can ensure that some of the guarantees of rust are enforced by the hardware. (It's been a while since I watched that talk, so I don't recall exactly which ones). And this is a microkernel, not a monolithic kernel, so they will be using hardware guarantees even between kernel components.
To be fair, 1) Zephyr can take advantage of an MMU if you have one, and 2) Linux itself scales down surprisingly far. Keep in mind that its lineage extends far back in time and that it retains much of its ability to run on low-spec hardware.
Linux required MMU from the start, uCLinux and nommu patches does work. I used it a long time ago it was great, but there are lots of small things like stack size is set compile time, memory mapped io does not work... You do get lots of functionality from Linux from my short three day experience with Zephyr I like uCLinux a lot better at least on STM32.
Xous seemed very nice together with Precursor which is the only platform I have seen it run on
