I am quoting from The Art Of Disney, Copyrite 1973:
"He (Walt Disney) died 6 months before the earth moving equipment arrived at the site, but he had already supervised the planning of Phase One down to the last detail and laid down the general principles for the development of the whole property"
In other words, He did know about the Magic Kingdom theme park and had a good idea of what it was to look like and what it was to contain. He died in 1966, 5 years before the park opened... It was not all designed after his death...