It works because liability coverage should be basically the same for both vehicles, and insignificant compared to non-liability insurance for a new car. The latter is not a legal requirement, but it is much more necessary for a new expensive car because it would be much more difficult to cover the cost of replacing such a car out of pocket if something happened to it. If you can cover the cost of replacing your car yourself, then you can avoid paying the insurance company a premium for their service.

For these reasons these types of insurance are better considered separately, and the legally required insurance won’t affect the conclusion of the analysis of which car is a better financial decision, so it is reasonable to omit mention of it.