While it's interesting to see a more social side to octopuses, calling this a city and comparing it to Atlantis is just silly. There is a lot that we don't know about these creatures, and it's exciting to observe new behavior. It's not so fun to see it through the lens of Quartz level science "journalism" however.
15 is not a small city. And the article never mentioned in what way the site is "engineered". I suspect the answer is "not at all." It's just a place in the ocean that octopuses like. Apparently octopuses found to tolerate each other gets turned into "octopuses built a city underwater" in the hands of unscrupulous authors.