Strictly the 'efi' folder is not needed since it's in the efi.img.
However, converting .iso to USB stick is a lot simpler when the
EFI folder is in the .iso too. This allows just copying .iso contents
to the USB stick and EFI boot will work. While Rufus supports
extracting the boot files from efi.img, it was considered more
portable to add the copy. This also simplifies updating setup-bootable
to prepare EFI bootable disks.