alt Hacker NewsX screens are legacy. It used to be you could connect to a particular screen by number to open windows on that screen but the modern way to do it is to have one big virtual screen. X screens were like having a separate X server for each monitor, but with a single shared cursor and shared VRAM. You can see why that's an obsolete model.
Yep. When did virtual screens come in? My last full time experience with X was with Xsun in the early 2000s under Solaris. There was a shared cursor, I thought you could drag windows between monitors, but I also thought the DISPLAY variable was different for each (though I could be misremembering)