Did the historical Buddha really teach all of the Buddhist practices that exist today? Not really, not especially Tantric Buddhism. Suttas (Pali for Sutras), which are closest to the historical Siddhartha Gautama, are also later compilations, so hard to say what exactly the Buddha taught. India was a fertile ground for Tantric cultures during the period when Tantric Buddhism, Kashmir Shaivism, and Kaula Tantrism evolved and got perfected. A lot of these techniques and deities are similar/shared because the true origins of some of these primordial entities/practices are older than known religion or metaphysics. Each philosophical school adopted them and claimed them as their own. The Vedas do not prescribe deity worship as practiced today either.
Buddhism chose to give a Buddhistic flavor to these Tantric/Bon practices by pinning Dependent Origination as their philosophical basis. The same can be said about Hinduism, where a lot of deities outside the Vedic systems were assimilated into a Vedic/Veda-friendly system that gets called Hinduism today. Hinduism also counter-attacked the Buddha as an imposter, a demon in some places, while later Puranas accepted him as one of the incarnations of Vishnu. Hinduism certainly is a much larger umbrella system as two dozen schools of philosophies exist (monism - not monotheism, dualism, qualified dualism, materialism, atheism, etc.)
So yes, a lot of these techniques are either very ancient or innovations of some later great minds. Either way, they work. In some areas, like practical aspects of non-duality, the Tantric Buddhists have perfected the techniques in a more elaborate fashion than the current day Hindu counterparts. It is a complex subject with many dimensions - history, society, philosophy, metaphysics, and then aspects that are beyond the scrutiny of the mind-body complex. I study these aspects academically but shun them away from my head when I practice with a clear head. While religion believes in constancy, preserving the old, purity, etc., Tantra (which is Shamanic in nature), is all about evolution, innovation, using the old to produce something new, and not getting sucked into the duality of good-bad, pure-impure, dark-light, etc., as everything that exists is a useful tool for self-improvement.