not exactly like that, but close. When you boot into the GUI installer, you are booting off SD card. So if you run this dpkg command it will install the packages on the current rootfs which is on the SD card, not on the eMMC.
however you can do something like this instead:
mount /dev/mmcblk0p9 /mnt # assuming p9 is the rootfs on eMMC (i don't remember OTOH)
cp /var/cache/apt/archives/xxx.deb /mnt
chroot /mnt dpkg -i /xxx.deb
with this command, you would in turn install the packages using a chroot into the eMMC rootfs.