That's interesting. A plausible theory, but not definite.
Mark 9: 4 And there appeared to them Elijah with Moses, who were talking with Jesus. 5Then Peter said to Jesus, “Rabbi, it is good for us to be here; let us make three dwellings, one for you, one for Moses, and one for Elijah.” 6He did not know what to say, for they were terrified. 7Then a cloud overshadowed them, and from the cloud there came a voice, “This is my Son, the Beloved; listen to him!” 8Suddenly when they looked around, they saw no one with them any more, but only Jesus.
It looks to me as tho Peter was confused or startled by the whole event and Peter, out of his discomfort, was joining in the conversation, rather than answering a question.
Then again you could look at Mark 9:2 "Mark 9:2 And after six days Jesus taketh with him Peter, and James, and John, and leadeth them up into an high mountain apart by themselves: and he was transfigured before them."
We note Jesus specifically selected Peter, James and John to accompany Him up on to a high mountain by themselves. Did they climb in total silence? Or on the way did they talk and perhaps Jesus told them what was going to happen? Even forewarned it would still be an overpowering and frightening experience to not only see Moses and Elijah appear but for Jesus Himself to be transformed before their eyes and for the Lord in heaven HIMSELF speak to them personally.
The point I am making is that we must be careful not to try and "mystify" everything in the Bible. To turn everything into an inexplicable "supernatural" experience. The Bible is an account of REALITY, not the "supernatural". Both the reality we know and that vast part of reality we do not know about except that the Lord reveal it to us. In truth there is no such thing as the "supernatural", it is all REALITY, it is just that mankind knows so little about the reality he lives in.
It is like the word "impossible". The Bible does not recognise the word "impossible". It does not say that the Lord can do the impossible. It says that with the Lord all things are POSSIBLE (that is, nothing is - impossible, there are just things WE know how to do and things we do not know how to do). How could the Lord create the earth as we know it in 6 days? The correct answer is because He KNOWS HOW to create the earth in 6 days and we do not. Is it possible to create the world in 6 days? Yes, because the Lord did it.
How did Peter know it was Moses and Elijah? The most LIKELY and SIMPLEST answer is that Jesus TOLD them who they were going to meet (or introduced them). Jesus did not take Peter, James and John up there to confuse them and frighten them. He took them up there to witness with their own eyes, not only His true identity, but the REALITY of everything He was teaching them. I am certain He would have prepared them for what they were about to witness. Do you think He would have done that?
We should always be careful not to overcomplicate that which is not complicated.