The stone in the cart that is wheeled up to the oven and the stone placed in the oven are round. And the stone when the pizza is removed certainly looks rectangular to my eye. However, to get clarification, I sent an email to Ms. Carter to get clarification. If she responds, I will update this thread. In the final analysis, I am not sure that it matters all that much whether the stone is round or rectangular although I would think that a rectangular stone would make for a slightly more enclosed mini-oven.
As for the characterization of the pizza as "Margherita", restaurants and chefs routinely exercise culinary license when making pizzas under that name. I have become accustomed to that to the point where I hardly pay attention anymore. I just read the particular restaurant's description of it before deciding whether to order it. Or, if I am suspicious for any reason, I will ask.