"More serious trialer" is not a fixed attribute or category that you can select for. A person fits this description based solely on their behavior; they become a more serious trialer by the act of putting in their email address. From that point the causality is the same.
Yes - my point was that the narrative of "why" isn't useful. A/B tests tell you what happened. They do not provide insight into why. Yet everyone has a story they tell to fill in the why, and these stories are harmful to making good decisions.