A nice story indeed, but utter bullshit. It has enough snippets sounding vaguely plausible, and similar enough to real facts, but assembled in a way that makes it wrong. To get info about the real deal about the A20 address line, check Wikipedia instead. Interestingly this wikipedia article is also linked from the "nice story" article.
Here's where the "nice story" is wrong:
- The keyboard controller is actually located on the motherboard, and available even if no keyboard is connected. Its job is to talk to the keyboard, but it's not part of the keyboard itself
- the A20 issue is not at all due to the BIOS memory test
- Re-enabling wrap-around for real-mode programs doesn't involve any computing power anywhere. So, no need to find a co-processor that is "idle", you only need to find one with a spare I/O line
The real explanation for the strange error message is actually the following: "No keyboard (or broken keyboard) connected to the computer. How could anybody possibly use a computer without a keyboard? Please connect a (working) keyboard to the computer, and Press F1 when done".
It's still stupid, but for a different reason (servers don't need keyboards). That's why modern BIOSes allow you to disable keyboard check, if you want to deliberately run your computer without a keyboard.