It could mean nothing, or it could not. But a small effect size alone doesn’t tell you this, and you are correct that there are many factors — that is exactly t

It could mean nothing, or it could not. But a small effect size alone doesn’t tell you this, and you are correct that there are many factors — that is exactly the point of this study, to tease apart which other factors are causative. Of course, this study says nothing about what happens if you try to change behavior, that seems like an entirely unrelated point. Taken together, I’m not sure how you can be so confident that a small effect size therefore means nothing.