People still think chocolate is something of an aphrodisiac now, it's not just some old timey notion. The reason you've never noticed that effect is likely to be any of the following on their own or in combination with each other. The purity (milk chocolate has less actual cacao in it than dark chocolate, for instance). Other ingredients interacting with the cacao (the stuff mentioned here didn't seem all that special but that could also be the author's fault for finding the wrong recipe if this one wouldn't actually work). The placebo effect (let's face it, most of the things people call aphrodisiacs just... aren't, so if they seem to work for some people that's possibly some mind over matter shit). The individual taking them (how many variations of anti-depressants do you think there are, just because no single one of them works on every single patient?).