There's no reason to think they lie more now about using a legal-for-adults substance than they used to lie about using an illegal-for everybody substance. Rather the opposite, if anything.
Or maybe they get a good look at their pothead parents and vow never to end up like that.
My parents were both chain smokers. It’s why I never took up the habit.
My point is that our society does not value truth anymore. I would bet that a lot of teens lie on these surveys just because they can. There is no benefit to them to answer truthfully. Personally, I would assume that any “anonymous” survey can actually be traced back to the individuals who took it - especially government surveys.