I think Wada is being a little bold with these predictions here, but I do see both sides of the argument. Firstly, Wada does recognize that a new Final Fantasy on a new console is always going to pique people's interest. Secondly, the battle system is something different while also being turn-based, and a lot of people I have talked to will not touch a JRPG unless it's turn-based.
Considering the possibility that people don't care about FF13, that could be because of the prevailing attitude here in the States. People don't want RPG's, they want blood, gore, and violence. Everybody knows that Final Fantasy will sell in Japan, and with all the fanboys here in the States, you know it'll sell at least one or two million. I have heard rumors about a PS3 FF13 bundle, so that will inflate the sales numbers a bit if its true. I think this game will his the 6 million unit mark eventually. Definitely not in a year or two if it took five years for FF12 to hit 5 million units.