Here's the deal: WE DON'T KNOW - Ergo, we should not presume. That includes just making stuff up to explain it.
We have no idea what happened at the transfiguration - whether it is tied to the future... whether the figures present were temporarily transported from the past... whether it was a vision supplied to the disciples simultaneously... We can pull stuff out of our hats all day long, speculating until the cows come home... But that is all it will be, is speculation, because what it is (transfiguration) is not well defined - and if we trust YHWH, it was intentionally left undefined.
It is one of the irksome characteristics of the Roman church to define things it knows nothing about - It seems there is no end of things which are defined from whole cloth, in a similar fashion to this example... Pulling an argument from a few disparate verses and building whole classes of doctrinal clap-trap right out of the air - Whereupon everyone must needs believe it simply because it has been ordained by some hierarchy of stuffy old men.
In almost every case, no proofs are required of them, nor any reliance upon the things which have come before (which all things are required to be derived from). It is just proclaimed as if it were fact.
it is not fact. We don't know. And that's OK.
But, see, we are constantly challenged about the Assumption of Mary because it is supposedly unScriptural.
Yet, we have this evidence in Scripture that say it is not.
Elijah was taken into heaven, bodily and without suffering death.
And, now in the New Testament, we have Moses with Jesus and Elijah.
Therefore, Mary being in heaven with her Son, body and soul is not unScriptural.
It is not presumption.
Unless, you can provide an explanation for how Moses appeared with Jesus and Elijah at the Transfiguration.
NO, not a ghost. And NO not a new body, which by the way is totally unScriptural.
So, how? Why?
We don't know. And that's OK.
We don't HAVE TO KNOW. We're capable of saying that we don't know but God does and that's good enough. Having to know reveals major control issues that we have to KNOW and explain EVERYTHING.
A person who is a control freak isn't really trusting God. They feel like they have to be in control because they don't trust anyone else to do a good enough job,(even God) or to do it right and that also includes having to be able to explain and understand everything.
It kind of fits with any religion where people feel they have to add works to their faith. They have control over their works and that way they can feel in control of the situation. It can be unnerving to turn total control of one's life over to someone else, even when that someone else is the God of the universe, who knows the beginning from the end and is more capable of making sure everything turns out right than we are with our limited knowledge.