But that doesn't mean they had the same conscious state. Language shapes reality as well as the other way round. Our ideas about ourselves -- the fact that we can talk about having "a self" -- is made available to us by language that might well not have been available to ancient peoples.
Indeed, and this is actually a really big issue and is really hard to get into. Something I remember especially from McCulloughs First Man of Rome were the latin jokes, written in English, which was very clever.
But if people think differently, I would put it down to cultural differences, though, not to one of level of development of consciousness (though I admit I may completely misunderstand the argument).
Even still, I would definitely agree that being unable to cover cultural attitudes and differences properly is going to undermine historical fiction. I suspect this is part of what the original argument was about.
And Porius sounds like a very interesting book for seeing this in action.