alt Hacker News(Total guess) this feels like an attempt at restricting what parts of the hardware pipeline need to know what; if prefetches need to key off of both the device bit and the nx bit, then both of those need to be piped into the frontend, whereas if device is only relevant for data prefetches then only the nx bit needs to be available there (while device would be piped into the backend where the data prefetcher lives).
Not really.
To the front end, one needs to pipe only appropriate logical combinations of the flags that describe the memory properties, not the flags as they are stored in some control register.
So a control register should contain flags that are meaningful for the programmer, while the kind of flags that hardware happens to need can be generated with a few logic gates from them and routed through hardware wherever they are needed.
It makes more sense to classify the memory in a few types, which must be specified by the programmer, including a "device memory" a.k.a. "memory-mapped peripherals" type, instead of having to specify for each memory area a long set of attributes about each kind of access that may be allowed, or not.
This is how it is done in x86-64.