No, it's aliasing, just as any fork() is. COW lowers the cost, for Mill as for any other. The Mill fork duplicates the address space, not the memory. The memory is duplicated page by page on demand, i.e. copy-on-write. Mill paging is quite conventional; it's the address space (SAS) that is different.
They were light on the details, but what stops an OS from mapping the same physical page into two distinct local regions of the address space and implementing copy-on-write as usual?
It's not, it's "copy on reference".