This is a good question. A little historical information might help you. H. G. Wells studied under the noted scholar Huxley, who was a proponent of social Darwinism. Moreover, it is good to realize that the social context in which Wells lived was infused with the ideas of social Darwinism.
Here are a few ways in which the idea of Social Darwinism plays out. First, there is a sense in which the Martians are superior humans. In fact, there is a learned reference to one of his former essays about Social Darwinism ("Man of the Year Million"). Here is the quote:
"It is worthy of remark that a certain speculative writer of quasi-scientific repute, writing long before the Martian invasion, did forecast for man a final structure not unlike the actual Martian condition..."
He, then, goes on to say that the Martians are more evolved than humans.
Second, based on the first point, there is a sense that there are lesser and more superior beings. Through this idea the notion of natural selection comes out. So, in the end, while Wells' work is not about Social Darwinism, it comes out as he is a product of his time.