Could you give me the context in which this claim is written. THe paragraph before and after?? A lot of times people take words out of context to make a point that is not true (not always intentionally)> This is known as 'quote mining'.
Unfortunately, you can't. There are not extant works of Thallus. You don't know what Thallus really said, you only know what Africanus said Thallus said, and Africanus had a political point to make.