borders have largely had guards, who had no obligation to let people through, or even to treat them fairly and not rob them. frontiers had bandits who existed solely to prey on travelers.
even if you could move to a different settlement, you did not have the same legal rights as citizens of that city.
the "modern passport" has done more for free human movement than anything that came before.
What kind of tech do you imagine is needed? Have you heard of Hadrian's Wall? The Great Wall of China?
Or more generally: forts, outposts, coastal batteries, scouts, patrols?
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.
What part of "If you moved from one place to another, you were mostly at the whims of thieves and pirates." suggests 1900 CE to you? Have you never looked at a history book?
Normally when someone says "1900" they are referring to 1900 AD. Unless BCE had already been mentioned, which here it had not. And if they were referring to 1900 BCE they normally would specifically say 1900 BCE. That's why.
And furthermore, the parent comment above the one talking about "pre-1900" was talking about modern passports. Why would anyone immediately jump from modern passports to 1900 BCE? That don't make no sense at all. Jumping to 1900 AD however, that does make sense. You see?
"Cities had guards.", "you were mostly at the whims of thieves and pirates."
What in that speaks to 1900 CE over 1900 BCE? There is evidence of pirates in both time periods, so that feature alone isn't telling. But the context doesn't end with that feature in a vacuum, does it?