There's a link on one side and a meta tag on the other. It's as simple as you can make the validation between two sites. It's not even fediverse-specific really - there were other services doing something similar before.
It's because freedom and correctness is hard. Yeah, most people prefer convenience and would rather someone be the source of authority to do it for them, but people on fediverse are not those kind of people.
Looks like it's as complicated as a parts inventory system developed in house for a half a million employee company...