Well, if nothing else, the design of the Mill has a much smaller "attack surface". There's not nearly as much in the way of interesting activities going on behind the scenes as compared to a modern OoOE CPU. That logic is pushed out of the hardware and into the software development toolchain.

The Mill has many other aspects that improve the security story. All protection is based on memory address, for example. And pointers are not integers. There are reserved portions of the address space that programs are completely not allowed to touch.