The answer to this can be found in the very first paragraph of the introduction to this work. There, Hobbes argues that the leviathan is artificial because it is made by people to imitate something in nature.
Hobbes argues that anything that people make to imitate nature is, by definition, artificial. He says that the leviathan is artificial because it is made by people to try to imitate a human being. He then goes on to list various parts of the leviathan and to note which parts of the human being they correspond to. As a couple of examples, he says that the sovereignty of the leviathan is its soul and the laws are the will of the being. There are also more physical parts, such as the government officials, who are the joints of the being.
Because the leviathan is made by human beings in this way, it is an artifical human being on a very large scale.