Surely one of the golden rules in marketing is to not alienate your audience. MrBullterrier, surely you'd agree that even if it is very minor, it's not exactly smart to take cheapshots at the people you want to sell a product to.
It's not a cheapshot, nor do they alienate their audience.
Are they trying to insult Americans? Are they trying to portray them as ignorant fools? Is the Ottoman Empire some commonly known empire, such as the British Empire? No.
Either you know about it, in which case it
doesn't apply to you and you should just go about your life confident that you're a part of the niche of Americans who know about the Ottomans; or you don't know about it in which case it's true.
You're reading too much into it, methinks.
Now, whether or not it's relevant is something else. Especially considering that the Songhai, for example, are much less known, and it doesn't mention anything about not many Americans knowing about it.
edit:
It's also worth mentioning that many of the gamers who're going to buy Civ5, have experience in playing other history-related strategy games, like, among others, Civ4, wherein they're included and the gamers therefore know about it or have heard about it. These people (a big, fat chunk of their audience) aren't included in the "most Americans"-group because they know about it, and they won't become upset at the comment unless they are overly-sensitive.