As you’ve no doubt guessed already, meringue is a terrific insulator. It’s made of thousands upon thousands of tiny bubbles, and each one of them works like a little air gap, slowing down the transfer of heat from the oven to the ice cream. Still air, you see, doesn’t move heat very well.
Thus meringue is great at putting the hammer down on convective heat transfer, i.e. movement of heat via air flows (or liquid flows). It’s not so good at preventing the other two major modes of heat transfer: conductive (objects of different temperatures in physical contact with one another) or radiant (electromagnetic radiation). The good news there is that most of the heat transfer that occurs inside a home oven is of the convective kind. Clever fellow, that Rumford guy.