The fact that the title screen, loading screen, cutscenes, etc. look correct in 16:9 are a huge case.
The title screen is a small thing, as is the cursor and inventory screen.
The loading screen, how do you know it's "correct" in 16:9? It shows just as much in 16:10 as it does in 16:9, only it's not stretched out as much.
And the cut-scenes mean nothing. There are SCADS of games out there that use 16:9 cut-scenes.
4:3 doesn't look right
How? Aside from the title screen, which is quite a tiny part of the game (no, coming first does NOT make the title screen a "massive" part of the game), how does 4:3 not look right?
one of these days you're going to have to open your eyes and realize that not everything is 4:3.
I'm aware that not everything is 4:3. I've been aware of this since Condemned: Criminal Origins. When it's clear that a game uses 16:9 as a baseline, I accept that. You have not proven that Trine is an example of such a game beyond the insignificant title screen.
It's entirely possible that due to a lack of communication, an artist made a title screen in 16:9 but the programmers assumed it would be 4:3 and programmed the title screen to scale accordingly, which is a more plausible explanation than asserting that a title screen proves the entire game revolves around 16:9.
Sure, some games are still designed purely for 4:3, but it's extremely rare nowadays.
Games that work fine in 4:3 but work even better in widescreen, on the other hand, are not rare at all.