According to the idea, the hybrids would have been gestated artificially, in vitro, by the aliens, then, after a successful hybrid had been created, it would have been possible (so the idea goes) for normal interbreeding between the hybrids and Earthlings to occur.
It's a plausible explanation...
The idea is that the human race is a hybrid consisting of an advanced (genetically superior) extraterrestrial race, and the crude semi-humans that were already here.
Announcing this would destabilize society.
There was an interesting quote from a Senator, who seems to be privy to some of this, a rambling statement that was mostly redundant and meaningless - the way someone speaks when he's restrained by requirements of secrecy, but he did say one thing that caught my attention, "Do you really think...
That is the dilemma of the human race.
Were we a genetic experiment that failed? A mix of ape-man with super-humans, making human?
Would there be mass chaos, if Humankind was informed of this?