It's a direct quote from one of the scientists:

'"I was absolutely stunned when I first saw the results," Prof Chris Griffiths, a senior author on the study, at Queen Mary University of London, told the BBC."'

Smokers don't magically undo the damage they did to their lungs after they stop smoking. Stunned might be hyperbole, but it is at least a little bit surprising.

I'd like to imagine the stunned scientist is a scientist from another field, they show some medical findings to a guy who looks at snail butts all day and he's the one "stunned" by the findings