After the survey I've become confused about what it means for HBD to be false. Should any difference between two separated populations be completely environmental? I believe it's an antiprediction to think it's not. I would bet that the "genetic potential" for any complex trait will be slightly different on average between different populations even if we are talking about two neighboring cities. Even if they started out as copies of each other just a few generations ago.

This looks to me like you have a fairly good view of what it would look like for HBD to be false. That is, there would be no meaningful biological diversity among humans, and the idea of people with different levels of intelligence would be as outlandish as the idea of people with different number of eyes. I mean, we don't expect that to vary between two neighboring cities, and even though there are some people who have something other than five fingers per hand, that's also something we would expect to only barely vary between cities, and so on.