Word of mouth is what will sell this game, I think.
Put it this way: If its as good as it looks, me, Gelion, Tekee, and several others who frequent civfanatics are going to be raving about it in the dedicated PI thread there. I know for a fact a fair few copies of HttT got shifted that way. Someone will post a screeshot in the alt history media section of althistory.com, people will see the astoundigly good map, get curious, and buy a copy. Rinse and repeat.
In fact... it might be well worth some of the PI marketing crew's time to register in those forums and do some stealth promoting (a concept I normally find stomach-turningly loathesome)...