Whilst it is restricted, you're not correct that it can't do unsound behaviour and can't do syscalls, and can't do non-determinism.

It can call unsafe blocks. They are more limited unsafe blocks, but they are still unsafe blocks.

I'm pretty sure I'm not, but feel free to make an actual demonstration to the contrary...

Unsafe blocks doesn't imply access to undefined behavior, merely the ability to write code that would be undefined in the regular non-const execution model.