To answer your question though in case the article didn't, actually maybe a better way to say it is this. You have a screen that is physically 2560x1600. However, if you decide to set 2880x1800 as your "scaling resolution" in System Preferences basically what it does is that it tells the OS to pretend the screen resolution is 2880x1800. Of course, you will see it "downscaled" to 2560x1600 because your screen can't see it in more pixels, but your operating system will act as though you have a higher resolution display, so Netflix will also think you're on a higher resolution display—just like YouTube and any other program.Why would Netflix think is a 2880x1800 display? It's a 2560x1600 one, so why the jump?
Also, why put a 2560x1600 resolution display if MacOS is limiting me to 1680x1050?
The more I read, the less I understand. In Windows it's very simple: you have a native 1080p display, you choose a 1080p resolution for it, and everything is 1080p in it.
In my MacBook/MacOS, my screen is native 2560x1600, the interface is at 1680x1050 if at max scaling, and Netflix is displaying at 1920x1080, even if it thinks is 2880x1800. How does that make sense? I must be very dumb, because I find it very confusing.
Windows actually has this as well. I forget exactly where it is, but you can choose scaling that is different from the normal display scaling (it shows up as a slider from 50-400% or something like that).
You said Netflix is displaying at 1080p. It probably "knows" that your resolution is set at 2880x1800, but it just displays 1080p to save bandwidth.
If you want more explanation about how it works on macOS feel free to ask and I'll do my best to explain what I know.