You are right that Nous mangeons du chocolat is often what you want to say – for “We are eating chocolate*.

But Nous mangeons le chocolat can be used at least in these two cases I can think about:

• le chocolat refers to one of these chocolates in a box. There are 20 chocolates in the box and we are eating one (OK it’s strange if many people are eating one chocolate (not one each, one for all)) ;

• le chocolat refers to a precised piece of chocolate, the one you are talking about, the bar of chocolate you just bought, the leftovers of Easter eggs in a bowl, anything you could describe as the chocolate – Did you really eat all the chocolate?!.