What is wrong with the Greek idea of antipodes that Lactantius argued against?

Lactantius did not see the earth as flat physically; he simply did not know, and he answered using the Greek antipodes idea. The antipodes reasoning assumes that if people stand upright on a flat surface, their heads are up, but in space there is no up or down; so the argument was that you could not walk on the opposite side of a flat earth without walking upside down, foot to foot. Lactantius said that cannot be, because God made man upright. Now some say he took the spiritual idea, that God made man upright meaning good, as it says five times in Genesis that God made things good, and wrongly turned it into a physical uprightness, and from that concluded the earth must be flat. That reasoning is not kosher, and I am not defending him in that. But I would say Lactantius is fighting against the antipodes idea more than he is arguing for a flat earth; he only reached the flat-earth assumption through his mistake with the antipodes, so his motive was not pure and his conclusion is in question.