I went on the website and this method is well known. Although I'd like to add some insights:
Never ever trust a gambling site made on wordpress when it comes to money (they don't ask you any money here, so no warnings).
It is not guaranteed, but the betting exchange on Betfair has always been a good way to make money.
You don't actually need to open any other bookmaker account, Betfair is sufficient since you buy back and lay (buy and sell) any bet.
The business model of this website is to refer you to a bookmaker site to open an account (which you dont need) and get them a commission.
It's not easy as it seems. You need to find & calculate the good odds for both outcome to cover your money. It's not a common win/win.
Finally, Betfair is only for UK residents I think.