The problem with that Wiki Answers link is that it cites "The Dictionary of Word and Phrase Origins", by William and Mary Morris, a source that is nearly always wrong. I believe they were just making stuff up. The best source for word origins is "The Word Detective", a weekly column (with an online archive) written by Evan Morris, who is, oddly enough, William and Mary's son. He cites his parents' theory, but in a polite way says they were full of
fertilizer. That's often the case when he cites his parents. Unfortunately, he doesn't really have a definitive answer, but you can read what he has at:
http://www.word-detective.com/030698.html (search for Cloud Nine).