Agreed: Our earth-based intuition is probably pretty off when the atmosphere creates 1% as much friction as it does here.

I doubt the debris field would be 1km in size. Assume you throw a high-speed small piece of propeller tip debris at the optimal angle at 240 m/s or about 800 fps. I'm sure it goes farther in lower Martian gravity and atmosphere but a spinning lead air rifle pellet at that speed and the optimal high-arc trajectory could barely make it half that far on Earth. None of the ballistics tables have columns for Martian atmosphere or gravity. (Yet! I hope I live to see a day when we could use this information, but not quite as much as I hope that we never have to use this in anger.)

But even in the worst case, you're scattering a 1.8 kilogram robot (more accurately, a few grams of the rotor blades) over an area of pi square kilometers. The chance that a large chunk hits Perseverance just wrong is negligible.