May I ask where you came by this information? Or is this how you would do it?
The amount of civs (18) has been confirmed. Judging by the civs chosen in previous installments and the nature of the series, they'll try to include the most influental civilizations. Also, what alternatives do they have? 18 civilizations of varying importance, 18 civs that never became great but deserve another chance so they get to be in Civ? Not quite as feasible if you ask me. Firaxis has to have some criteria for their choices, and historical significance is likely the most significant, judging by their track records.
Also, 16 (at least according to a
this thread) of the civs have been confirmed already. They are America, Germany, Aztec, Japan, China, Arabia, Mongolia, Rome, France, India, England, Songhai, Russia, Ottomans, Egypt and Greece. They match the criteria I proposed perfectly: They have been dominant factions that other nations feared, envied or admired. The only one of these that surprised me somewhat was Songhai, but as I said, it's likely that Firaxis wants to keep the pack of 18 diverse. Egypt as the only African civ would leave the majority of the African peoples unrepresented, and Songhai balances this a bit. However, Songhai was no weakling either - it was an important trader empire, and among the largest of African empires.
So, answer to your question: I came up with this information by studying the confirmed information about Civ V and by observing the very nature of the series. Picking strongest civs is also how I would do it myself, since I see no other way that would result in similarly pleasing gameplay.