Cities had guards. There was no border pre-1900 because there was no tech to enforce it. If you moved from one place to another, you were mostly at the whims of thieves and pirates.
Agriculture, at very least. Before we created that tech you'd be way too busy trying to find something to eat to have time to stand around defending artificial borders. 1900 BCE mightn't be perfectly accurate, but close enough for a stupid comment on the internet.
Nothing in the comment that we are talking about indicates that "pre-1900" is referring to 1900 BCE. It sounds squarely like it's referring to 1900 AD to me. Which is why people are saying it's ridiculous.
