What I meant by "the intervention is closed borders" is that closed borders require action by the state, open borders are the "do nothing" stance. This seems pretty clearly true to me.
I think you are arguing against my point that open borders are the rule and closed borders the exception. It's true, most countries adopt some form of restriction of movement. But that does not negate the fact that most movement people make (between cities, regions, etc) is not restricted. And as is mentioned in the post, they were not restricted historically either.
Yet one could still argue that "the rule" is this very specific combination which we find in countries today of practically unlimited movement within borders and difficult movement across borders, and then invoke a precautionary principle. But that sort of thinking would imply a rejection of basically any proposed change to the status quo. I don't think anyone would propose consistently following such a conservative framework for policy implementation and if they did - ironically - I think the result would be a level of policy paralysis which would be disastrous.
Do you favor any policy changes? If you do, can you affirm that it 1) has more evidence in favor of it than migration; and 2) could not conceivably have disastrous results?
"We should be extremely humble about our views on complex topics, and the downsides of open borders, if we are wrong, are quite significant"
But this argument cuts both ways. The downsides of closed borders are also quite significant. However, the intervention is closed borders, not open borders; the evidence is on the side of open borders, not closed borders; the vast majority of worl's movements are not across borders, so free movement is the rule and restricted movement the exception. Why would a "precautionary principle" favor the case for closed borders?