Saying it's like "adding a new swap file that is only to be used for the contents of that specific file" isn't really correct. Swap works by letting the kernel

Saying it's like "adding a new swap file that is only to be used for the contents of that specific file" isn't really correct. Swap works by letting the kernel write a memory page to the swap file/partition. After it has done that, the memory is marked as backed by a filesystem (i.e it's no longer anonymous), and the region of physical RAM can be repurposed for something else. With mmap'd files, the memory is always backed by a filesystem; there are no writes, and the physical memory can be discarded at any time.