I see a few possible explanations there. Please, take with a grain of salt.

First, smoker's lung/lung cells are "cleared" more often due to harmful effect of smoking. This is why long-time smokers have morning coughs and why some life-long smokers eventually get nasty diseases. But it may be beneficial short-term with nCoV.

Second, smoking/nicotine changes something in the lungs at biochemical level. I remember talks that smoking constricts smaller blood vessels. This alone may be enough for the immune system to fight the virus faster.

Third, and I think the most probable one. Our bodies are trained to the stress and bad stuff in our lungs, so... virus. what virus? We did not even notice any virus and become asymptomatic/mild cases.