I don't know where you got five years from. Five years ago tablets (as we know them know) just barely existed. Even today it's not like the competing split-screen options are setting the world on fire. They range from mediocre (Windows) to horrible (Samsung). I don't know what Apple will accomplish, but it's hard think of them as behind when the current implementations are mostly junk.
Windows multi screen is awesome. You can have more than 2 windows, they resize quite easily with a fingertip, etc. If you want even finer control you can go crazy with windows in desktop mode. The advantage of this is that when you dock your windows tablet then you can manipulate windows in complex ways since you are on a real computer. I'm only guessing here, but whatever janky method Apple uses in iOS to multitask will be crappy when hooked up like a desktop, if that's even an option with the non existent mouse support.
And you're right, 5 years doesn't make sense. I think it's probably closer to 20 years. Tablets did in fact exist before the iPad. Certainly Apple's genius marketing made them mainstream, but that doesn't make them revolutionary from a technical view.