Another possible motivation may be that he doesn't believe that any of this is actually alien technology at all, and that it is in fact a skunk works program he either has stumbled into via some side channel, (and therefore doesn't have direct access to materials that with substantiate his testimony), or he may even know exactly what all this stuff is but can't speak frankly about it because it is protected by top secret protocols that trigger his immediate sanction if abridged, cutting off his only means to publicize them further.
In either of these cases if he has decided that these programs are an existential danger to the world and need to be discussed, one way to out maneuver both of the above constraints is to pose as someone who believes they have been convinced the government is concealing "alien" technology, and can use his credentials to push the story farther than any civilian could. He has found that this tactic has given him access to people he otherwise couldn't reach (outside the limited context of his position/rank etc.)
tl;dr talking candidly about actual top secret research because he suspects it may constitute an existential threat to humanity if it is developed much further (similar to some early physicists' speculation that testing the H bomb could trigger the earth's atmosphere to burn up as well), that could get him in serious trouble very quickly because he's clearly crossing a line that has been clearly drawn around that information. But revealing it in terms of alien tech, he'll either get dismissed as a crank after his 15 minutrs are up, or he'll have gained access to high ranking civilian legislators long enough (with whom he can discuss the actual details directly in private), until he can find enough sympathetic ears that he feels safe to dispense with the "omg alien tech" camouflage.