if it woud come from a guy like shinobi, i would believe it but i never heard of that guy leaking something, so i dont really believe it and why would Microsoft "ruin" a what seems to be like, an insane press conference by announcing a price of 499 Dollar!?!? And like i said, a 300 Dollar price difference i just dont believe that, 399 Dollar will be a perfect competitive price but we will see.
He's not a random online guy like shinobi. And he doesn't make a name for himself leaking things. He's the one who is up on stage hosting game award shows. He's the one who gets to interview world famous developers and get exclusive information, etc.
This is not in the same league as random online leakers.
As for why Microsoft would "announce" the price? Their conference is 5 hoursd away. There's only so much they can do to avoid information spreading at E3. Things like this get out. Final Fantasy 7 Remake got out around a day before it was announced. Silikonera staked their reputation on it saying they knew for sure that it will happen. And it did.
Well ok, we will find out everything in a few hours anyway if hes right, i still believe in 399 Dollar but that can be totally wrong of course.