This explanation is poor. The keyboard controller and interface are crucial. NKRO is possible, but can it can be flaky over USB. There are many variables involved.
Want to learn more? Hit the geekhack forums (search for NKRO) or geekhack channel on freenode.
I wouldn't say the explanation is "poor." The article details one of the reasons behind the lack of n-key rollover. Note how the text explicitly limits the scope of the article.
> Typically, ghosting is the result of one or more of following three limitations: the hardware can't read the given key combination, the software on the computer doesn't support multiple simultaneous keys, or the communication protocol between the hardware and software limits the maximum number of simultaneous keys reported. The next section discusses in more detail the hardware design of typical keyboards that limits the number of keys that can be read at the same time.
Want to learn more? Hit the geekhack forums (search for NKRO) or geekhack channel on freenode.