If you have a belt that move perpendicular to the print head, then the extrusion path is in in the plane of the of the belt. If you try and print something that is longer than the travel path of the print head then it will have to stitch the end of the object with the new material as the object moves off the end of the belt.
By being at an angle, the print head has full access to the entire end of the object that is being formed. Think of the point where the print head makes contact with the object surface as the extrusion plane. The belt and printing at an angle enables the extrusion plane to never be occluded by the part itself. It is like an infinitely tall hang printer that can move up forever, only it is on its side so it can continuously bond the print belt and the part.
This design allows for parts to be made that fill the x-y projected area of the printer while being arbitrarily long in z, the belt can feed into a tray that allows the part to rest at the belt height. You could use this to make forms for sail planes, foils, spars, etc.
I can't wait for this to get out into the hands of the mhackers to see what they come up with. The next phase is when something like this is multi-material, so one can have say a water soluble support material and an impregnated nylon in the same continuous part. As it is now, the primary purpose would be for small scale manufacturing and cosplay swords (like the demo).
Think of printing an 8' long rectangle that is 2 layers high.
If you print diagonally, the cross section is always the same.
If you print on a level, you get the 8' length divided into smaller sections that are pieced together. The cross sections are not equal.