We have a load of trouble this past winter season with trickle ventilators being draughty when closed! I would not be unreasonable to think that when closed trickle ventilators don’t let in draughts. BUT A LOT OF THE TIME THEY DO!
Here are two both less than ten years old and both draughty, note the dust staining. The first one had never been opened, the householder didn’t even know what it was!