It's really not all that silly. As I said, Walt Disney did investigate the option before his death.
If he chose to exercise that option, I can imagine the family choosing to tell the general public that his remains were cremated. Again, the family very much values its privacy and dignity.
Just look at the comments made in this thread that were in questionable taste. If the family were to have said that Walt Disney's body was frozen in 1966, the public response may have been similar.
The other consideration is that certain members of the public have a difficult time accepting the loss of beloved figures, so that phenomenon may have also given rise to the rumor.