Officially, yes, you’re are absolutely right, but in Walt’s head, it was the Magic Kingdom. He even referred to the lands as “Realms”. For what I understand the name Magic Kingdom didn’t have much pull, because back in the 50’s, what was the meaning of MK anyway? It meant nothing. So I believe it was Roy who wanted a strong brand, something familiar to millions and that was the Disney last name. Disneyland, everyone knew that name… also back then they didn’t know they were going to keep expanding, which it opposite from the Florida Project. That’s why they made sure they introduce the land as WDW and the park as MK, because there was more to come. But even today many tourists still referring to MK as Disney World or just simply Disney, even though there’s a huge MK sign at the entrance. I guess the decade it took to open Epcot Center absorbed the MK name once again.