That's an incorrect assumption
That's what I'm saying - the numbers in the Wikipedia article don't work, therefore some assumption they made is wrong. If you look just at the Wikipedia claim (800,000 members, of whom 20% contributed > 1 million posts) and fill in an upper boundary on the total number of posts (today's number, 1.45 million) then I'm saying those numbers don't work.
All I'm talking about here is the internal consistency of that claim - I don't know who's contributed what to DW, but I know that that quote doesn't make sense. So if you're also saying that the Wikipedia claim doesn't make sense, then I'm relieved, because it means my math is probably okay.