> What, specifically, makes you think it has something to do with the time leading up to 1900 CE rather than 1900 BCE
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?