JS wrote it ambiguously enough to permit others to come to that conclusion.
It's an assumption on the part of mormons to believe that. JS never recorded that he was speaking to God or that Jesus was speaking to him.
The use of the word "Personages" is in no way an adequate description of God and Jesus, I don't care how you try to spin that.
1 John:
4:1 Beloved, do not believe every spirit, but test the spirits to see whether they are from God, for many false prophets have gone out into the world.
2 By this you know the Spirit of God: every spirit that confesses that Jesus Christ has come in the flesh is from God,
3 and every spirit that does not confess Jesus is not from God. This is the spirit of the antichrist, which you heard was coming and now is in the world already.
4 Little children, you are from God and have overcome them, for he who is in you is greater than he who is in the world.
5 They are from the world; therefore they speak from the world, and the world listens to them.
6 We are from God. Whoever knows God listens to us; whoever is not from God does not listen to us. By this we know the Spirit of truth and the spirit of error.
First off, the warning about testing the spirits and the fact that false prophets would be known by their worldliness. Well, JS was quite worldly. He was a lecher, a thief, a con-man, etc. Many of the things JS did while he was the "prophet" were the behaviors that are in direct contradiction of a Godly man. There were a lot of "convenient" "revelations" that "permitted" him a great deal of latitude.
Secondly, the "personages" description and the failure of the spirits to clearly identify and confess Jesus during the purported 1st vision.
The narrative (1st vision, [pick a version]) is ambiguous, it does not clearly state that God and Jesus, came down, nor does he claim to have heard the name of Jesus, just "this is my beloved son..." (That IS NOT "confessing" Jesus). It's implied through the use of descriptive language. But at no time does the personage(spirit) state absolutely that it was Jesus who was going to "speak" to JS.
Wishing something to be true doesn't make it true.
The mormons HAVE to believe JS was telling the truth or the whole, vile, scam comes crashing down around their heads. The Bible stands on it's own, the BoM not so. The D&C, no chance, a collection of heresies and blasphemies written by a lying, adultering, thieving person of absolutely no good character.
--MormoNdude(It just took him a REAL long time before he decided to share that test with then rest of us.)