This may indeed be true of the religion as established by Gautama and more or less practiced in various places still today.
It's hardly true of Tibetan Buddhism, which has many thousands of deities, ascended Buddhas, demons, heavens and hells, etc. This is largely because it is a fusion of the indigenous Tibetan "pagan" religion, Bon, and the imported Buddhism.
It may be true they don't believe in God, in a monotheistic sense, but they certainly have more than enough gods. The more Buddhist-influenced believers consider these to be forces, influences or metaphors, much as the more "advanced" Hindus do, while those more influenced by Bon tend to view them more as actual entities with real existence.
In fact, in some senses, the Dalai Lama is himself considered a god, a reincarnation of the God of Compassion.
None of those deities is considered to be self-existent. Your entire screed is in error. That is not Tibetan Buddhism.