Apparently it is not a Tomb Raider game, CD have left the name |"Tomb Raider" out of the title deliberately, and it is more arcade style, incorporates another playable character, a Mayan Indian, also going to be a short game with approx only 6 hours play.
More info in this Tom Raider Forums thread click here
I have played all the Tomb Raider games up to Anniversary and have just started Underworld. I'm also playing Tomb Raider Chronicles (the fifth game)for the fourth time as I play from TR1 upwards every year or so. I'll definitely download Gaurdian of Light.