To push back a little: I don't think this is true for all success stories, and although this is true for some core organizers, many eventual organizers were participants in our discussion group/community events first, and then eventually became organizers (which in turn resulted in the outcomes described). You're definitely correct that some of the folks though are definitively NOT counterfactual (e.g. Will, you, me) and were already taking EA taking actions without the group's influence.
To clarify, I don't think that events are low impact - they may very well have the harder-to-measure forms of impact you're describing here! Mostly I'm trying to draw a comparison to some of our activities, like our discussion group, which were lower effort to setup and had clear, measurable positive outcomes.