Have a game that's profitable enough to pass the threshold and then interest drops off? You're suddenly incentivized to completely take it off of the market and remove the game from people's libraries since you'll keep racking up fees from installs even if no one ever buys another copy.
I'm not denying that the quote does seem to imply what you're saying, but I have to believe that's a misprint or bad writing on their part, the implications of the threshold being lifetime sales are so bad. The policy is bad, but there's no way Unity is that comically out of touch, is there?