Yes, looks like she used a baking sheet, but I'd use a 9x13 too
I still think of baking sheets as a cookie sheet with sides because when I was little they were both cookie sheets to me. I'd want a thicker cake, so like Lisa, would opt for the 9x13 or 2-3 round cake pans depending on how much batter I ended up with.
That finished picture of the one in the baking sheet looks like a bit of a sad puddle. All that mint needs is a chocolate kiss on top and the cake would look like a melted Wicked Witch of The West - might be perfect for a movie night that included the new Oz film.