As they were coming down from the mountain,
he charged them not to relate what they had seen to anyone,
except when the Son of Man had risen from the dead.
So they kept the matter to themselves,
questioning what rising from the dead meant.
Of course Peter and John didn't understand anything that happened on Tabor. How could they? Rising from the dead had to be a metaphor because it simply could not be literal.
This is one of Jesus' things he does: say things and mean them. And it's one of our things we do: hear Jesus and think he must mean something other than what he plainly said. I think we can be forgiven for our confusion - a lot of those parables are kind of opaque. Narrow gates, who understands 'em, eh? Amirite?!