Robert T Ricketts wrote that article after a failed attempt at trying to keep a mbu long term in a 180 gallon tank. After his single experience with attempting to keep his mbu in a 180 gallon tank, he made a post about what he personally felt the fish truly required (1,000 gallons) which has then been repeated over & over & over again ad nauseum.
Stuart Morse later echoes the same sentiments in his article on Mbu, after his failed attempt at raising his Mbu in a 100 gallon tank. (400 litres to be precise)
And Pufferpunk, while you may not have originally come up with that number, you have probably repeated it more times than anyone else.