It was not hard to believe at all. Zemey Rinpoche stopped teaching immediately after this book was published and entered clear light immediately after as it was published by one of his students. It is horrible to see a student betray his own Lama in this way after all the years of care, love and guidance because of personal vendettas. I am pretty sure the stories were meant to make Gelugpas not indulge in non Gelug teachings, but the way it was written and translated was distorted in a way that it makes it look odd. I am sure there is a commentary to it but it is omitted on purpose for obvious reasons.
Do you have the sourse for this information?
The Yellow Book was written in 1970 and published in 1973. According to the text 'Selected Writings of Kyebje Zemey Rinpoche', Kyabje Zemey Rinpoche taught extensivley from 1970 until 1984 at Gaden and at various other places in India and Nepal. He then retired from public life due to a stroke he had in 1984.
Kyabje Zemey Rinpoche himself chose what to include in his collected works, which were published in 1994.
The Yellow Book can be found in volume 2 (Kha).
I am sure that it was never intended for general readership however; like many Tantric texts it must be understood in context.
Much like Shakyamuni when he declared "Kill your mother, kill your father", if we take that at face value we are in trouble. It is the underlining meaning of the words that must be tapped into. If all of the Tantras were picked apart and taken as literally as the Yellow Book has, there would be no end to the misunderstanding and confusion.
As for the student who helped Rinpoche publish his texts, Geshe Thubten Jinpa has never 'betrayed' his Guru that I am aware of. My Teacher here at Zemey Labrang speaks very highly of him.