I don't know what you mean--there's nothing highlighted in that link when I open it. Part of it is about elastic collisions and part is about inelastic.

> the issue is that this is modeled as a very sturdy wall anchored to the Earth.

That's not what you modeled in the comment you linked. Your math (incorrectly) attempts to describe a car colliding with another car of equal mass. But if you're considering a "very sturdy wall anchored to the earth", then you effectively have a car colliding with the earth.

Either one is fine with me, but pick one. It grinds my gears when you authoritatively call me "absolutely incorrect", "correct" me with an incorrect answer of your own, and then try to claim you were modeling something entirely different all along.