Reagan used a common expression of how something outside of humanity that would threaten all of humanity would unify humanity into a common cause.
It is a common way of trying to convince people that we can unify today and have peace today if we just look beyond our differences or arguments.
Reagan was not warning us that Space Aliens are coming.
“Reagan was not warning us that Space Aliens are coming.”
I think you are right; however, he did mention it as a possiblility when he was negotiating with Gorbachev in Iceland.