App tells customer to review it. Customer reviews the app honestly (the nag is annoying and deserves 1 star). Seems like the nag screen worked? :)
(Although in seriousness, I get this point they make further down in the article: "If we don’t request ratings from all users, then the self-initiated ratings tend to be disproportionately people with a complaint rather than satisfied users. ". That's a real problem. )