It looks like to be a common habit between the industry. Like Activision requesting to MW2 the title of best selling game of all time. 

S-E may start to say that FFXIII is going to beat MW2 record, or that it will be the game to revive the Japanese industry(well, this one is ok), to be the game that will make americans love RPG as they love FPS and europeans to love RPG as they love soccer. Things we know that want happen, but just the hype around can sell some copies.

Now S-E says that it will be the best 

I doubt FFXIII's total sales will reach MW2's first week total.

it most likely wont reach MW2 sales first week (even though they should) but it will certainly be wayyy better than MW2

I'm not getting what you mean. Did Square-Enix actually say anything like that or are you theorizing that they might?

Given the marketing push they will give it, anything is possible.

Since they didn't really say anything like that my answer to the title is no. They did say they want it to be the best selling FF title which isn't too outrageous of a prediction since this one is on two primary home consoles as opposed to one.

The fact is, S-E is hyping the sales of this game. I'm just theorizing the situations, and the second one is already told and the first one have a chance of being actually announced.

Another question

If they spend the same amount in marketing as Activision did with MW2 it may reach the same result? Or S-E would have to spend the double to sell the same?

I remember seeing this:

Yoichi Wada: When we say that it is going to be the best selling title ever, the bar is really high because of Final Fantasy VII sold 10 million units. So is it okay to think what you are going to be asking is that it is going to be selling a lot?

What I got from that is Square Enix hopes for it to be the best selling Final Fantasy.

They did also say it is the most anticipated game of all time in the US/EU release date trailer.

S-E did say 6 million in an article, tho I cant seem to find it. Ill put it up when/if I do.