Where is that metal sheet you mentioned? On my boat there is traveler rail and tiller goes under it and then into the rudder head. If the boat capsizes, rudder cannot slide off the pins since tiller is held down by the rail. Tiller itself just slides into the rudder head, but there is a line from the rudder that is cleated to the tiller so the tiller itself cannot slide out of the rudder head.
See this photo from Wayfarer site: almost identical, except that Wayfarer uses pin instead of line to secure the tiller.