That would be true if they were measuring sales numbers, but they're asking people if they smoke, which includes those who smoke contraband cigarettes.
"oh yes I buy smuggled goods" said no one ever to a formal survey.
Why would you have to say where you get them at all? It's not like they were asking about an illegal drug. What would be the point of lying about whether you smoke?
They're not asking where the cigarettes are coming from.