I think you're reading far too much into things with this take. As far as I know, Windows also tends to treat Intel integrated graphics as pseudo-discrete. Intel always wanted others to take better advantage of unified memory, but faced an uphill battle because the performance ceiling was so low, nobody really wanted to bother. Easier to treat them as a crappy discrete GPU and be done with it.
Also if you go back far enough, Intel integrated had limits on how much RAM the iGPU was allowed to see, usually 2GB or less. And even though they've lifted such hardware limits in their modern iGPUs, Windows still only lets the iGPU see and use at most half of system memory. This means drivers still sometimes have to copy data back and forth, and if you don't do things the right way you don't end up with shared zero-copy buffers. That's why that Intel document talked about the need to allocate the memory with a special API.
The same kind of thing should be possible in Metal on macOS AFAIK, regardless of Apple's overall advice "just treat it as if it's discrete". It never actually is discrete, it's just a question of how large the graphics aperture is (that is, the zone of shared memory), and how the APIs work to allocate memory in that zone, let general purpose users take some of it for non-graphics purposes, wire some of it down as GPU memory, optimize away copies when possible, and so on.