Yeah, it's just impossible to know how biased this interpretation or the initial statements from the professor were.

Lots of things like these are discussed in the same way as physics professors say: "Imagine an infinite frictionless plane" - that it's a model for thinking about concepts and not a policy plan.

Imagine a completely borderless world where the only goal is literally economic efficiency - capital and labor are able to migrate freely anywhere and anything less is harmful in a strictly economic sense.

Practically, that's a bad idea for lots of reasons (like it might be economically efficient for someone to murder me, but I'd find it rather inconvenient) - so maybe some rules about passage of criminals and you can go on from there.

The standard example is Michael Clemens, who argues the total cost of closed borders to the world is in the trillions of dollars. If you want an article reviewing more views: https://www.washingtonpost.com/news/wonk/wp/2016/10/14/why-e...

The basic argument is just the standard free market argument. if A would like to sell their labor and B would like to buy it, and the government intervenes to stop them, that destroys whatever value they could have gotten from the exchange. The fact that there are some border markers between A and B doesn't change that value, although it may introduce externalities.

It's essentially a weird labor market protectionism policy, and really no economist argues protectionism is good.