A describes a pretty common flaw, that failing to make and argument does not mean the conclusion is false. In this case, that is exactly what the author does. If we grant her the idea that the stones do not prove that the builders were good astronomers, does that guarantees that they were not good astronomers? Of course not. Just because you disprove one line of reasoning for a certain conclusion does not mean that the conclusion is false, but just the argument is flawed.