That was after they got non-GRRM writers to finish up Season 8. One of the running themes Martin peppered throughout the SOIAF was to set up and then crush classic fantasy themes that didn't play out in history. Load-bearing Bosses and magic with no ontological inertia were right up there.

So I suspect there might actually be a zombie king, and he may even be hard to kill. And killing him will be about as effective in stopping the zombie army as killing an officer in a human army.

Some people still hope that Martin will finish Winds of Winter and Dreams of Spring and he won't feel obligated to mirror the TV series. It's a far reach, but a hope.