[I]t is debatable whether Chih-i ever conceived the idea of one single Buddha, or found it meaningful. There is, in fact, a fundamental difference between the doctrine that “the three bodies are one body” and the idea that “all buddhas are one Buddha only” (issaibutsu ichibutsu), which would later be put forward in Japanese Tendai. Chih-i acknowledged, and justified, the existence of other Buddhas, and did not eventually reduce them to Śākyamuni Buddha (they are not Śākyamuni’s upāya). In the last analysis, Chih-i regarded Śākyamuni only as the most important Buddha of the Lotus Sutra and only as the Buddha of the present world. He claimed that the three bodies all reveal the “origin,” but he never qualified this original time as the absolute time. His “origin” is just the archetypal movement, the attainment of buddhahood.
A Buddhist Kaleidoscope; Lucia Dolce, Between Duration and Eternity: Hermeneutics of the ‘Ancient Buddha’ of the Lotus Sutra in Chih-i and Nichiren, Page 229