The reason Apple gave is the truth, LPDDR3 only supports 32GB. You would need DDR4 for 64GB but Intel only supports regular DDR4 not LPDDR4. While DDR4 uses les

The reason Apple gave is the truth, LPDDR3 only supports 32GB. You would need DDR4 for 64GB but Intel only supports regular DDR4 not LPDDR4. While DDR4 uses less power when active, LP saves significant amounts of power during standby and they would need bigger batteries to compensate.