I mean, it's not like people could've ever gotten the idea that he was NOT a European cat who gives a miller's son riches in exchange for boots. ;)
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Nathaniel: Sire, do you...like yourself?
Edward: What's not to like?
The reason for the title could be 2:
1) Perhaps director and producers didn't want this movie to be related with other versions of the "Puss 'n Boot". Versions that kids have seen lately (like the one that appears in Shrek). Besides this movie is more faithful to Charles Perrault's original tale. There are some differences here and there in order to make it more kid-friendly (i.e. in the original tale the 'puss in boots' did devours the ogre once he convinces the ogre to transform himself into a mouse, while in the movie the ogre transforms himself into a swan and isn't devoured, etc.).
2) Lots of stories written in the XVIII century and before, used to include in its title the words "The True Story of...". That was just a custom, though. it doesn't mean the authors really intended to tell the "true story" as we understand it nowadays. Probably the creators of this flick liked the idea of using and old-fashioned title, and so it was called "La véritable histoire du Chat Botté"
cheers.