Call me a wannabe-aboo if you want, but I've never seen Attack on Titan. All I know is that it's the one about naked giants who eat people. Apparently, in between being gobbled up like Saturn's son, the characters find time to visit a fancy restaurant. Given the task of filling the backgrounds for that scene, someone put a screenshot of some sailing ships from Total War: Empire in a frame and knocked off for the day.
It's the first screenshot you see on the Steam page for Total War: Empire, which makes this even funnier. Given the working conditions at animation studios in Japan, I'm not bothered by someone taking a shortcut rather than creating an entire Renaissance artwork just to round out a few shots. It's just funny that it's a screenshot from a videogame, and a particularly blatant one at that.
Normally the kind of references used in anime are a little more obscure, or obscured. For instance, when isekai series The Eminence in Shadow needed a train station, the artists redrew Flinders Street Station from Melbourne, Australia. The funny part of that is they kept the anachronistic LED screens, and you can even see the machines where you top up your Myki (Melbourne's public transport card, like the Oyster card in London).
It's far from the first time videogame artwork has been reused out of context. In addition to all the fake war footage made with Arma videos, there was the time a trailer for Uncharted 4 reused concept art from Assassin's Creed 4: Black Flag. That led to Naughty Dog apologizing and updating the trailer with new artwork, but in this case I hope nothing gets changed and it remains as a fun Easter egg for future viewers to find.