I have never tried the freezing cookie sheet to flatten a warped mat, but I did successfully flatten one.
I put my warped mat out on my back patio on a very hot hot day. I laid the mat directly on the hot concrete, laid a black cookie sheet that was slightly bigger than the mat on top of the mat. I then weighted the cookie sheet with bricks all over it.
I left it there until the next morning when everything was cool. Removed the bricks and cookie sheet and the mat was flat and has remained flat. Perhaps either one is as good as the other!