In reading the Shamatha section of Dudjom Lingpa's Vajra Heart Essence
and Alan B. Wallace's commentary in it, it is said that Shamatha alone can also lead to a certain level of clairvoyance. Although of course that in itself isn't Buddhahood, and given the nature of Vipassana (special in-sight), the latter would be more conducive to higher levels of clairvoyance (clear vision).
Anyway, fact is that all major religions have their occult or esoteric aspect, and Buddhism is no exception.
Many exoteric Muslims think that Sufis are shirq or kafiruwn, many exoteric Christians think that Gnostics are Satanic, and many Hinayanaists (exoteric) think that Mahayana (mesoteric) and Vajrayana (esoteric) were not taught by the Buddha Shakyamuni. Of course Vajrayana was not taught publicly
by Buddha Shakyamuni, like Sufism wasn't taught publicly
Even Jesus said in the exoteric gospels that he taught only the exoteric dimension of his teachings to the profane and that he reserved the esoteric teachings for his disciples. I seem to recall that there are similar instances in some Sutras (Mahayana and non-Mahayana) where reference is made to Buddhism having a more esoteric dimension (Mahayana and Tantrayana) which many Vaibhashikas and Sautrantikas won't accept. Regarding that, what Pemachophel said.
By the way, the Vajrayana Masters are very capable of defending the positions of Mantrayana using pure logic and reasoning (without having to only appeal to authority & refer to the more so-called "fantastical" aspects of Tantrayana) such as seen in the links in both my previous post and in my posts on page one of this thread.