You've lost me with the non 3D comments, it is 3D from what I can see your character can move around the whole map and move up and down the ramps, buildings, walls railings etc. That is moving in three dimensions, they've just moved the camera a bit higher.
If they manage to include half the fun of the Dreamcast game it'll be a killer game.
The screens you see before you of Jet Set Radio for GBA are called 'isometric'. They're different from 3D because the objects on the screen are 2D sprites... Not 3D rendered meshes.
Of course, people fairly new to computers wont understand that, not to be insulting, so i'll reiterate.
A 2D sprite is basically a series of flat images, each image a frame of animation for a particular movement. These are usually layed over another 2D 'enviroment' which again is just a flat image, although they can be used with 3D backgrounds.
3D meshes are actually wireframes, images which appear to be made of wire, usually in 'net' form, that is, layed out flat. The 3D renderer in the game will take these, fold them out into 3D objects, and then fill the gaps between the wires with textures to make it more realistic - textures being flat images, this time layed over a wireframe form instead of left by themselves.
This basically all boils down to a few main facts. Firstly, a 3D image can be rotated in any way by the rendering software without requiring animations for the job - this is why you can move the camera around in 3D games like Sonic Adventure or Crash Bandicoot, but can't do the same in 2D ones like Mario World. 3D graphics generally also look different - around the edges they usually have a slight stepping effect, where the slanting edge might look a bit like some steep stairs, but they usually look more real... Like they're popping out of the screen rather than just lying flat.
This game's basically just made out of totally 2D components, with the world slightly tilted - Meaning it's basically 2D, like all of the Tony Hawk's games on GBA. Although, to make things more confusing, the Tony hawk's GBA titles have used 3D characters and 2D backgrounds - a bit like most of the Resident Evil games.
Hope that clarified things for you.
Oh, and as for that post about comparing a 3D JSR game to Driver 2 - I did say it could've used Raylight's BLueRoses engine, which has some very impressive results so far, with playstation-like graphics. I wouldn't call that pretty ugly, and it proves that with enough effort the GBA can be used to make fairly impressive games, rather than the half-baked mish-mash it usually attracts.