“This impact was the size of the Earth, and the size of the rock would have vaporized any of the inner planets.”
More accurately, the mark left after the impact was the size of Earth.
Shoemaker-Levy left similar (or larger) marks. It wasn’t enough to “vaporize any of the inner planets”, and neither was this. Something that size would have been a major impact event on Earth though.
It does give one good example of why we need an active space program and why we need to get comfortable with space travel. We need the ability to move those that will hit earth to a different orbit.
Its easy to say that it doesn’t happen enough to worry about but we shouldn’t forget that the last one was only about 100 years ago. If an explosion the size of the Tunguska event were to happen in the midwest it would likely kill millions.