The term "ghosting" isn't being used properly. Ghosting is when you press 2 keys, and a 3rd key appears on the screen that you didn't press. That's the ghost key. It's extremely, extremely rare to find ghosting on any keyboard at all; controllers are designed to block all possible combinations of keystrokes that would result in a ghost key being sent.
The term that refers to how many keys you can press at once is rollover. But the reason that companies like MS, Logitech, and Razer don't use it because it's defined by the FEWEST number of keys you can press. Even if you can press up to 20 keys at once, if there's another combination of just 2 keys that results in blocking, it's still called 2-key rollover.
There are plenty of boards with full n-key rollover (as in, you can press every single key on the keyboard at once), but they are all mechanical. NKRO requires that either a diode is placed in series with every keyswitch, which isn't feasible on a membrane sheet. It requires a PCB.