My guy, this is making the opposite argument from what you think:

"On the illegal market, no one is checking IDs before selling marijuana. When and where cannabis is illegal, high schoolers often sell cannabis to their peers. In contrast, licensed cannabis stores have overwhelming compliance with age-gating."

It has indeed not increased the cannabis use of kids, but that would also still be illegal. That study is an argument that age gating works.

You would be surprised to know that illegal weed sells still happen where it’s legal because it’s cheaper.

But even if that wasn’t the case, underage cigarette smoking has been rampant forever