I've heard of people taking it, but haven't heard of any positive reviews. That website is kinda trash.
They claim this
"A study conducted on males of an average age of 40 showed that approximately 170mg of epicatechin per day, dosed at 2mg per kg of bodyweight, resulted in almost a 50% increase in follistatin and a 16.6% decrease in myostatin, alongside a strength increase of 7%."
When the study was only 6 males, and the 7% increase was in hand grip strength that was observed, muscle tissue mass was not measured, and there was no placebo group for comparison.
You could pick apart every claim that website makes if you just google the study and give it a once over but in general, "Due to the preliminary nature of studies, there is not currently enough evidence to support epicatechin for muscle growth. However, dark chocolate is safe and provides other health benefits, so consuming it regularly may not be a bad idea."