You can read his book Buddhism: One Teacher, Many Traditions
Sometimes people mistakenly believe that Tibetan Buddhism, especially
Vajrayāna, is separate from the rest of Buddhism. When I visited Thailand
many years ago, some people initially thought that Tibetans had a different
religion. However, when we sat together and discussed the vinaya, sūtras,
abhidharma, and such topics as the thirty-seven aids to awakening, the four
concentrations, four immaterial absorptions, four truths of the āryas, and
noble eightfold path, we saw that Theravāda and Tibetan Buddhism have many
common practices and teachings.