Just yesterday the question What does 'OP' mean? was asked, but it didn't need to be asked. The answer was on StackExchange just not on this part of the site. Stack Exchange Glossary - Dictionary of Commonly-Used Terms
It's unlikely that he would have searched here and found it by himself so,
What is the reason that you can't search for or mark duplicates between sites that may have your answer?
Basically, each site is part of the network, but they're considered independent entities - you can pretty much use one site and not ever touch the other parts of the network. There's no expectation that you will look at other sites, because the idea is that each topic is separate from each other. Marking things as duplicates cross-sites doesn't really make a lot of sense, especially in the case of meta sites, where the sites mainly exist to ask about, well, stuff related to that specific main site.
In this case, you could have quoted the relevant part of that post in your answer, and then linked to it/attributed it appropriately in case the OP wanted to know more of that sort of information.
There's no need to mark this as a duplicate of something on another site.
