I could discuss this for hours—but “anti-gravity” is a folk version of the technology allegedly being used.
The actual claim is that “portals” are created and the craft “skip” through them.
Since the “portal” is not part of our space/time it is not subject to our “laws of gravity”.
As to secrecy—it all depends on how you define the term.
There have been many dozens of folks claiming to participate in the programs.
The counter-intelligence “trick” is to minimize the number of “respectable” people who believe them.
The issue I have with Salla's work is that it is speculative and unsupported by a convincing argument—though that doesn't necessarily mean it's untrue. That said, if Salla has knowledge of gravitic propulsion and is still alive to discuss it openly, it raises the question of why even a small aspect of this 'technology' hasn't visibly influenced today's military advancements.