”It works” is relative and the statement that "St Mark's Square is pretty much dry" is a lie. It is flooded as every year, a friend is just there for a visit and sent a foto. Not knee deep, but more than 15cm. Could have been worse, I guess, but it's still flooded.
The article talks about that: the barriers currently go up when the water level is expected to be 130cm or more (above some mean level, I guess). St. Mark's floods at 90cm according to the article.
When there was a 132cm tide, the barriers went up and St. Mark's stayed dry. The next day, the tide was 106cm, the barriers stayed down, St. Mark's flooded. What was the exact date on your friend's photo? What was the tide height that day?
