But, no, that's not right. If someone is typically a buyer of a ... Range Rover, or a BMW X5, or something like that, then replacing that car with an EV of equivalent size makes a proportionally larger impact on the environment than someone who replaces their Corolla with a small EV.

Your missing the point. An EV is not necessary better than a gas car.

An EV like the Plaid X or the Hummer EV actually use the equivalent of more fuel than the competition. A gas X5 is more energy efficient than a X.