Finally finished the main story and upon seeing those three words, thought "WHAT!?" I was pretty miffed. I really, really enjoyed the game, which makes that statement even worse.

I'll keep playing and see what happens with the 160 artefact completion thing (not to mention what it says once the endless credits finish ) - oh, and what the official book says, as in the walkthrough & tour guide it doesn't say anything of the 'Continued' bit.

EDIT: Given that it's impossible to get all 160 artefacts before finishing your first finish of the storyline (some only appear after the final battle), I guess it's a guaranteed to be continued... so hopefully that's not to say that the 'secret' ending is still leaving you on a cliffhanger. Give me another day and hopefully I'll know for sure!