One layer is nowwhere near enough. He is using twill and if the weight is around 300 g/m2 6 layer is 1,6mm so one layer is ~0,27-0,35mm. Now imagine that you have so thin layer of glasfiber and you need to start sanding it smooth. Although the foam looks pretty smooth in the pictures there will come lots of holes to the capsule when you start sanding and adding plastic padding.
Holes in the capsule means water can get inside the structure and it will freeze during winter and cause lots of other problems. One layer might keep the body together if you driven nicely but if you want the rigidy of what regular cars have you need about 1.6-2mm wall thickness. plus extra on corners. You have to think about surviving in crashes althought that is not the main purpose of the vehicle, those can happen from many reasons.
I did my final thesis of that Citycab self supportin carbon fiber upper body
ERA has full carbon fiber monocoque. which was next car after Citycab and next after ERA has also similar structure.
On my lupo project I did a hood with 2 +2 layers of 300g/m2 twill and its flimsy. Its not the same as having that foam inside when you would have basicly 4+4 layers.
When using hand layup the structure wont become so stiff as when using vacuum pressure to suck the resin and pressure to push it againts the tool.