Even NUMA is a "flat space of contiguous addresses". There's no bank switching or whatever. Yes, the access times are non-uniform, but that's orthogonal. The point of my comment to the OP is that if you're going to be super-pedantic about this stuff, you need to get your terminology right.

Are we assuming x64? Many ARM systems have gaps in their physical address space. x64 can too but not always. For example the Gameboy Advance used the top 8 bits of the address to identify the peripheral that is accessed.