Reagan spoke several times about "an alien threat uniting us", unless I
misread the clip, Kissinger is saying an alien threat would make all
people of the world unite in a world government

Right, but Reagan used it as an illustration how petty our International differences may be. Kissinger was illustrating how easily we may be willing to forfeit our liberty while under threat . They both used the idea of alien invaders, however it seems to me they are talking about two different things. One is about uniting on the majority of things that express our similarities to fight a common enemy, the other is about forfeiting our liberty so that we may be saved. I don't know....just ...

Reagan spoke several times about "an alien threat uniting us", unless I
misread the clip, Kissinger is saying an alien threat would make all
people of the world unite in a world government

Right, but Reagan used it as an illustration how petty our International differences may be. Kissinger was illustrating how easily we may be willing to forfeit our liberty while under threat . They both used the idea of alien invaders, however it seems to me they are talking about two different things. One is about uniting on the majority of things that express our similarities to fight a common enemy, the other is about forfeiting our liberty so that we may be saved. I don't know....just seems like two completely different things.
It is an interesting thought. It seems to be kind of the point of the Global Warming fraud. The 21st century version of war socialism. It's not as dirty, but more people die.