The story of the aliens is an allegory for
The Overstory’s overall point about the nature of time. The aliens are like people, and humans (in the story) are like trees. Because the aliens move so much faster than humans and thus experience time in a different way, they assume that human beings lack any kind of consciousness (they’re simply “sculptures of immobile meat”). As a result, they feel no guilt about exploiting them for their resources. This, the book implies, is exactly how humans treat trees, simply because we cannot understand them. Furthermore, most people can’t comprehend the fact that what seem like years to us might only be seconds for a tree.