This is the correct interpretation - you get a bit longer max clockspeed due to the thermal mass (and thus more heat overall).

Is the added plastic shell case a "bandaid on a bandaid" sort of solution to deal with that heat? Absolutely. But you might want that case anyway - I've had several laptops that would have had broken screens or were yanked off a desk by an attached cable and survived by the sacrificial plastic shell taking the impact.

Like all things, it's a tradeoff to consider.