What problem is this solving? Are there no OSes for medium embedded systems? Are they too expensive?

Key aspects from the talk iirc (I was in the audience :)):

* Real time embedded CPUs are usually without an MMU -> kernels such as FreeRTOS lack secure memory due to the lack of MMUs in those CPUs

* A kernel targeting embedded CPUs with MMUs that supports secure memory management

* Secure memory communication a there called server/client method to communicate leveraging Rust borrow checker build time for later having "user-land processes" to communicate via pages.

These things combined allow a very small kernel, with user-space implementation of usually kernel-level functionality, such as the system clock timer (presented in the talk).

All of this is meant to provide a complete trustworthy processing chain, from CPU dies that can be inspected through infrared microscopy through the CPU epoxy package/cover to the entire build/software tool chain.

The Xous OS project both takes care of the Kernel, but also the CPU/RISC-V runtime with an MMU, something that is usually quite difficult to obtain - but due to synergy effects with another chip consumer/organization they managed getting their custom processor manufactured.

Trust and transparency: https://betrusted.io

There is QNX. seL4 is another.

The former is proprietary. The latter kernel is GPL2, similar to Linux.