The most important theme is not the assertion that there is intelligent life somewhere in the universe, but assertions about the nature of man. Man is essentially evolutionary intelligence. The earth is our "cradle"--we are in the infant stages of intelligent life. Only when we are able to make it to the moon will we demonstrate that we have reached a new stage of intelligence. Man is destined to evolve beyond the earth--to transcend the limits of earthly life and our "infant civilization." Therefore space exploration and travel is essential to human development--the sign that we are evolving into higher forms of intelligent life. The fantasy of some vastly superior form of intelligence--as represented by the Sentinel--is the fantasy about the real nature and future of humanity. "They" represent our possible future if we keep evolving, keep doing the space thing.
A sub-theme is the moment of speculation about "contact" with alien forms of life. These forms are clearly intellectually superior--but will they be benevolent or hostile? Part of the pleasure of this kind of story is the feeling that we will very soon find out.