Sorry to quote my own post, but I tried Tomb Raider Legend, Lord of the Rings: The Two Towers and Lord of the Rings: Return of the King today. All three are listed as widescreen compatible in the list, yet all three merely stretch the image and are thus by definition not widescreen compatible.
I bet half of the supposedly 'widescreen compatible' games in the list are like this.
That's actually a good idea, correct me if I'm wrong, games that support native widescreen have the option to switch it in game, it could be easy detect what games are actually using true widescreen. From the games I've tested so far, if they doesn't have in-game option for widescreen they only will strecht the image to cover black borders.
Not sure what are the parameters to submit a game as widescreen compatible in the list, but it "should" be as you say.