Yes, if they did not solder on the memory it would use more power. The longer the lines are to your DRAM, the more impedance there is and you need higher drive power on your memory controller. LPDDR has been soldered forever as far as I know, though with the introduction of CAMM (compression attached memory modules), this has changed. I don’t know but I would bet money CAMM is still higher power for less bandwidth than DRAM packaged on the SoC base die or however apples does it.