Parent is drawing a parallel. We collectively agreed that the tobacco industry played a role in the consequences of tobacco addiction by marketing to minors.

The parallel being drawn here is that social media platforms, being presented with the consequences of their products ie: teen mental health declines with use, are complicit in those consequences at some point in the value chain, either by marketing, product design, accessibility, etc. If the assumptions hold, then the conclusions are sound and valid.

There's two ways to disagree with this. Either the assumptions are not valid, or the argument is not sound.