Maybe what Tantra reveals is that the Buddha’s conclusions weren’t the whole picture, just one phase. Once the central channel activates, the actual mechanics of ‘higher’ consciousness (shiva/shakti, prana and citta) come online whether a philosophy accounts for them or not. Maybe Buddhism maps the ‘mind field’, while systems like Tantra, neidan, and Saivism address subtle infrastructure. So it’s not about belief, but about which circuitry is active and which field one is engaging.
The Buddha would have known about the tantric culture of his time, but he rejected it, and honestly I can’t blame him, as much as I’m interested in ‘tantra’ I’m not remotely interested in being taught tantra. But in the end I suspect he bypassed all subtle infrastructure and found an explanation that works philosophically but energetically leaves subtle mechanics unengaged.