"Open borders" was the historical situation for much of US history. It's comparatively recently that we've been closing them with anything like effectiveness. That said, the nature of "open borders" changes as transportation becomes easier, which it did dramatically over that period.

From the beginning of the post:

"One puzzling thing I notice about debating immigration is how certain people often are that strictly restricting immigration is the right policy."

He's talking about people who support significant controls on immigration, which is the current policy and the direction which some people want to move further in.

Lee's not just addressing people who want to end immigration. The substantive argument he's making is that significant controls on immigration are bad.

Isn't something like 40% of USA fortune 500 companies started by immigrants?

The right wing in the US has been programmed to be eternally afraid of the "other," regardless of what form it takes. Ingenuity is diminished when you try to filter out different perspectives.