Viggen has no INS, but you are correct. Remember, 'simulation' is a wide term. Not all modules (or possibly none of them) have what you'd call 'research grade' INS models i.e. you probably wouldn't use them to find out how an INS responds in situation XYZ, and since this is primarily a game, we don't really need that kind of thing anyway, just something that is close enough to fool us.