"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.
It's not a causality, it's a pre-filter. So, of the people who passed filter A more are likely to pass filter B.
Today's XKCD was basically about this (in the Alt text)