Because Satan in Heaven..is not "completely consistent" with anything I've ever read.
Read the book.
Draw your own conclusions. Satan’s existence was revealed to the boy whilst in Heaven. I would venture the probability that Satan was not in Heaven at the time.
He was revealed details of his parents life he could not have known about, and apparent details of his future, and the future of his parents. Satan was a part of that revelation.
Whether John was taken to Heaven in his revelation is unknown to me, but clearly details of the antichrist were revealed to him, wherever he was at that time as well.
Again, do your own homework.