No question that Delta has leverage in this dynamic with Korean, too, but despite what we've been assured repeatedly - that it was Delta calling the shots, and it was just a matter of time until Korean submitted to Delta's will - the reality has always been that Korean has its own "cards" to "play" as well. (And, in fact, several of us predicted precisely this - that Korean could and potentially would develop some form of relationship with AA.)
It's true that Delta is the second largest U.S. carrier to Asia - behind United - but Korean itself has the most extensive U.S. network of any Asian airline, so they're quite the "catch" for any prospective partner, too. Plus, the real strategic criticality of Korean to Delta - at least from what I can see - is that Korean offers extensive, fast and convenient connectivity over the exceptional ICN hub deep into Asia - cities large and small. As Delta's own NRT hub has been progressively dismantled, ICN has become ever-more-attractive as a jumping off point to access "deep" Asia.