I said that because it is recorded that way in Matthew 17:9.
As they were coming down the mountain, Jesus instructed them, "Don't tell anyone
what you have seen, until the Son of Man has been raised from the dead."
As they were descending the mountain, Jesus laid a command upon them. "Tell no one," He said, "
of the sight you have seen till the Son of Man has risen from among the dead."
As they were coming down from the mountain, Jesus commanded them, saying, "Don't tell anyone
what you saw, until the Son of Man has risen from the dead."
Even if you did not want to go by these translations which do not say 'vision', you still have not proven that what they saw being only in their mind.