That would be interesting, however after launching adb shell:

  shell@mako:/ $ grep vfat /proc/filesystems
          vfat
  shell@mako:/ $ grep vfat /proc/mounts
  /dev/block/platform/msm_sdcc.1/by-name/modem /firmware vfat ro,relatime,uid=1000,gid=1000,fmask=0337,dmask=0227,codepage=cp437,iocharset=iso8859-1,shortname=mixed,errors=remount-ro 0 0

So fat is being used in spite of not having an SD card.