Grim Fandango is a 3D game despite having a static camera because the characters themselves can rotate in 3D
Rotating the camera usually means the game is in three dimensions. I haven't seen a game in which you can rotate the camera but that isn't 3D. Just like you said, in this game you cannot rotate the camera, but you can rotate the character himself since he is a three-dimensional model. And the entire environment, as far as I understand (I have not played this game), is also three-dimensional.
What about visual novels labeled 3D? There are pre-rendered flat pictures, and also pre-rendered 3D animations (video). But there is no real three-dimensionality there. The camera cannot be rotated, and the character cannot be rotated. The characters are not three-dimensional models, nor are the entire environment. Roughly speaking, this is a set of pictures and animations (videos).
The problem is that tags are selected by the game creator, and every creator has a different interpretation of what the tags mean.
They have no alternative. The 3D tag has a lot of meanings, and naturally, when there is only one tag, they add it. But if there are two separate tags, 3D game and 3D computer graphics, it will become more clear where there is real three-dimensionality and where graphics created using 3D technology are used. Naturally, the game co-creators themselves will add tags to their games. But in this case, users will be able to complain about developers who have assigned an inappropriate tag to their game. Now there is nothing to complain about.