Mostly Urban Legend.

Early in Venus' history it was hit by plenty such objects, yet it still has a thick atmosphere, so something is wrong with the concept somewhere.

Things to ponder are:

Do the volatiles of a planet's atmosphere and hydrosphere escape slowly over geologic time, or in a rush at the beginning? I feel that it certainly front-loaded. The Earth is a bit anomalous because with plate tectonics it actually recycles volatiles back to the mantle so appears to produce them at a greater rate today than is actually the case. (Yes it produces volatiles from volcanoes etc., but they are not "new" volatiles)

Is impact erosion overrated? For Mars, this is certainly the case and the old models of huge impact erosion of the early atmosphere are undergoing drastic downscaling at the moment.

Finally, if we have the power to move such large objects around the solar system, what happen when Osama bin Spaceman gets hold of the controls?