Plates for Unused Outlet Box

I'm renovating my kitchen. The new dishwasher has the option of a "plug in" connection to an outlet. As per the directions, the outlet was moved from behind the dishwasher to the back wall of the sink. The electrician left a metal plate to cover the old box where the ends of the wires of the old outlet are covered with plugs. The workers for the contractors covered the outlet with "something" and plastered over the hole. They claimed the cover was so thick that it would push the dishwasher too far forward. This makes no sense to me because the power cord and hose are already on the back of the dishwasher and take up more space than the metal plate. What I'm really concerned about is whether they've violated electrical code.