There is a good book about buddhist siddhas in India by Keith Dowman: Masters of Mahamudra, it is worth reading. The thing is that some of the buddhist siddhas were adopted by different hindu/nonbuddhist siddha-lineages. This is one of the themes that Keith Dowman discusses at length in his book.
You can find mention, for example, of Gorakhnath in modern hindu tantra yoga movements. There are books about Gorakhnath or Goraksha in english. In one of the preserved songs of Goraksha, in the hindu tradition, he mentions Shunyata, this word has now been translated as "purity".
What do you mean by buddhists siddhas being adopted
by hindus or other sects? Adding their names to lineage charts or what?
So does the book say that Goraksha was a buddhist who was somehow taken to be a hindu and his teachings were interpreted according to hindu philosophy and not according to a buddhist philosphy which makes his texts warped? I remember a friend once telling me when reading some book of Goraksha that it just doesn't make sense, perhaps this is why.
It is clear from the buddhist sources, like the ones that have been translated bu David Templeman
, that Buddhism existed as an organisation, as a distinct identity, in India at the time of Buddhaguptanath, ie in 1500's. There is a difference between buddhism and hindu tantrism, as regards doctrine and as regards the principles in the religious social organisation that is called Sangha.
In modern european theistic society it happens that a person has been a buddhist and an atheist for the whole of his adult life, but when he dies his relatives arrange a theistic funeral for him!
In a similar fashion it is just plain evident that when buddhism ceased to exist in India as a religous movement that could defend itself, the hindus could decide anything they wanted about the buddhists that they decided to keep.
The fact is that same persons exist in the buddhist siddha lineages, these you can find in Keith Dowman's
books and on his website, and in hindu nath yogi and some other hindu lineages. If You didn't know, the hindus also adopted Buddha Gautama Shakyamuni at some point, they declared him to be an Avatar of Vishnu!
In Europe (and world wide) we have the case of Baden-Powell
, the founder of Scout movement: He hated religion in his youth, and escaped from the school during the lessons of theistic religion. It has been said that he never uttered the word that means "supreme spirit". During my time I have seen how the wikipedia article about Baden-Powell has been changed from a purely atheist themed biography to a theistic one! What can You do, if You are dead yourself? 'Evidence' can easily be forged, when the stakes are high.