"Ritual skull cups are traditionally formed from a human skull that has been cut into shape, lined with a metal rim and ornamented. Many skull cups are simply made out of a precious metal in the form of a cranium. They are usually elaborately decorated with artistic designs and Buddhist symbols like lotuses and vajras. Many are fitted with ornamented lids and have feet or a separate base in the form of human skulls.
As the libation vessel of a Vajrayana Buddhist, the skull cup can be seen as a parallel of the clay pot (kumbha in Sanskrit) of the Vedic sacrifice, the alms bowl of the Buddha, and the sacred water vase (kalasha in Sanskrit) of the bodhisattvas. In addition, as a receptacle for sacrificial offerings presented to wrathful deities, the skull cup parallels the tray of auspicious substances like jewels, flowers, or fruit presented to peaceful deities. In its most benign symbolism, as the begging bowl or food vessel of an ascetic, the skull cup serves as a constant reminder of death and impermanence.
I can't speak about thee magical potency of skullcups made from the different enumerated sources, but he is right that skullcups were made and used. This much is certainly correct. But as other posters have noted, Parenti seems to have an axe to grind, His work has been quoted by the PRC for example (which doesn't necessarily invalidate his observations).
Parenti believes that people project Shangri-la onto Tibet and wants to disabuse people of that fantasy.
That being said, what I believe is remarkable about old Tibet is that they kept the Olympic Flame of enlightenment alive as the entire world went through countless social, military, economic, and technological convulsions, almost like a time capsule with life inside it.
As Thurman has (perhaps somewhat over-)stated, Tibet kept the ideal of enlightenment alive. And some people also attained enlightenment (as they did all throughout the Buddhist world) down to the present. However Tibet was not immune to overwhelming negativities, war and murder. They also conquered the minds of people in Tibet and the Himalayan region just as they did throughout the world. The main difference is that in most of the world universal love, lovingkindness and compassion were at best ideals but were viewed as actually unrealizable. And in some of the world, values were definitely inverted, just like the cannibals on Cannibal Island are supposed to have done (the Aztecs, Nazi German & Japanese genocide for example but to a lesser extent all of Europe and the English world at minimum).
_________________Kirt's Tibetan Translation Notes
"Only you can make your mind beautiful."
HH Chetsang Rinpoche