A year later, and this is no longer true. Their website says that, as of January 2025, wifi is available on "B737-8, B787-10, A321neo, and A350-900 aircraft." This includes such routes as ICN-YVR (operated by a 787-10).

Additionally, it seems like some of their 77Ws now have wifi as well (though this is not mentioned anywhere on Korean's website), so you may get lucky on other long-haul routes. I flew a 77W recently on a Korean TPAC flight and purchased wifi from Panasonic ($21 for the full flight). Speed test showed ~21 Mbps down, ~2 Mbps up. The wifi was a lot faster and more reliable than the Panasonic wifi I've tried on other carriers (eg, TPAC flights on United and JAL).