I can't say I disagree with that, not that I would categorically lump all financial managers into the 'useless' category either.
But it is important to note that he uses 'trading' in the sense of actively taking and releasing positions in different stocks in order to capture their short term change in value. Not 'investing' which is buying and holding equity (stocks) in company that are growing or otherwise generating wealth (and ideally paying a dividend).