The main difference there is that the walls make it impossible to view the frame at an angle.
The problem here is that distance makes things smaller/bigger, so the side that is closer to you will appear distorted and bigger, which is what makes it impossible in the first place.
The second thing to note here, is that there’s clearly some distortion going on in that video, I tried for many days to make it work on SurfaceGUI’s, but without success.
What makes billboards special is that they always face the camera, meaning that there wont be any distortion. In addition, setting the viewportframe size to the player’s viewport size makes it so that it will always appear to cover the whole screen (when ClipsDescendants is off), which is one of the main requirements for portals, see here:
Also, when approaching the surface, the SurfaceGUI will have to shrink to a point before you actually reach the wall, or else, it will look like you are running into a flat painting.
Since the SurfaceGUI is prone to perspective distortion (The side further away seems smaller), it means it will never be able to perfect cover the whole screen, unless we get a way to distort it back, for example, by using shaders. This is the effect I am talking about:
Only if you limit the viewing angles like in the video, and even then, there will still be some distortion, plus the GUI will shrink to a point before you reach the surface, if you don’t do this, it will look like you are slamming into a flat painting.