I actually went and read the report... It says that "in the study, which included nearly 1,000 adults, the more chocolate the men and women consumed the lower their mood. Those who ate the most - more than six regular 28g size bars a month - scored the highest on depression, using a recognised scale."
Next, the report says that the scientists said "there are many possible explanations for their findings, and that these need to be explored". and that they "cannot rule out that chocolate may be a cause rather than the cure for being depressed".
"Many possible explanations". "Cannot rule out". These are tentative, heavily qualified statements. Nobody actually said that "chocolate causes depression".