The point is that if you attain dhyana, samadhi and samapatti you attain whole universes of knowledge.
No. Without vipaśyāna, dhyana, samadhi and samapatti just led to rebirth in deva realms.
This knowledge has existed much before what you call "the advent of tantra", it has existed from the beginning, This is the inner experience area, it is also the area of tantra. It seems that there developed some schools that consciously deny the inner experience aspect of Dharma, in name only they accept that there is "enlightenment", their "enlightenment" is without knowledge and without experience, only then is it "true" enlightenment.
In the history of Dharma there is inner experience from the very beginning, therefore there is also tantra, or the likeness of tantra, from the very beginning. But then there are people with authority and power who want to decide what "true " enlightenment is, and hence they start their own schools whose main purpose is to wipe out some irritating persons and their influence who actually attained dhyana, samadhi and samapatti.
I think that for example Buddhaghosha had a wealth of inner the Dharma, I would even say that innerly he was a tantric, this is because of the necessary dynamics of the dhyana process.
Is this so difficult to understand?
I understand your point of view, though I do not agree with it. I don't agree with your conspiracy theory either.
You seem think that Vajrayāna is all about dhyāna, samapatti and samadhi. It isn't.
Vajrayāna practice is a method of reversing dependent origination. Of course, in that process we need to use samadhi and samapatti. But not in the way it is used in sutrayāna methodology.
There is a school in Tibetan Buddhism called the Kagyu school. They teach a system called mahāmudra. They assert that mahāmudra is also found in the Mahāyāna sūtras (but not, of course in Hīnayāna sūtras. And they teach, in their mahāmudra, a system called sūtra mahāmudra which is the basis of the famed four yogas of mahāmudra of Gampopa. They emphasize the practice of dhyana in their system of mahāmudra
They also clearly recognize a mahāmudra that comes from Vajrayāna practice. They further recognize a type of mahāmudra that comes from sudden insight based on a kind of introduction by a master, called essence mahāmudra.
They clearly differentiate sūtra mahāmudra and "tantric" mahāmudra by the methods that are used. The methods used in tantric mahāmudra are things like creation stage (visualizing oneself as a deity), mantra recitation, working with prāṇayāma (not mindfulness of breathing), "erotic" yogas, yogas connected with sleep, waking, etc.
The methods used in sutra mahāmudra are solely śamatha and vipaśyāna combined with the four yogas -- which are really stages in the deepening of śamatha and vipaśyāna.
Essence mahāmudra has no methods.
Vajrayāna is different than sūtra teachings because the methods of deity yoga, prāṇayāma, etc., are never taught in sūtra. No cakras, no ṇāḍīs, etc. None of that newage hippy Vajrayāna stuff is found in the sūtras, nor explained by sūtra masters -- not in India, not in China, not in Japan.
We Vajrayānists assert that all of our hippy methods, deity yoga, and so on, cause our path to be faster than the pure Mahāyāna sūtra route. These criteria did not evolve in Tibet, they evolved in India. Of course, there is some internal debate as I mentioned above. The Kagyu school in many respects is close to Chan school and even uses some Chan scriptures as a basis for their arguments.
You can accept or reject Vajrayāna claims -- but at least you should clearly understand what they are -- a clarity that thus far has been absent in your presentation.