The AMA may have received comments form curious people outside of r/futurology since there was an announcement for it on the front page. One thing about r/futurology, too, is that it recently tripled in size - only a few months ago it has around 6k subscribers. A lot of the growth came a week or two ago from a thread featured on r/bestof that got a lot of attention. Those things probably contributed to the inferential distance... If the AMA had happened a few months ago it may have been less, or indeed if it had happened a few months from now, counting on there being significant attrition of those new subscribers.