16GB is the limit on LPDDR3 (not sure whether it's spec or Intel's implementation), so you can pick either battery life and 16GB with LPDDR3 or 32+ at the cost of battery life (and a mainboard redesign) with desktop-class DDR4.
The DDR3 spec limits each individual DRAM chip to 8Gb (Gigabits, not bytes) - 8 individual chips makes up a full 64-bit bus, so for 16GB of total memory Apple needs 16x8Gb DDR3 chips in the notebook. I'm not sure if Intel CPU's are limited to 16 individual DRAM chips when used with LPDDR3, but even if they COULD be used with 32x8Gb modules it would suck up a lot more power.
DDR4 allows FOUR times the capacity on the same amount of individual chips, instead of a full dual-rank DIMM capping out at 16GB like DDR3 they can be up to 64GB. On the MacBook line with LPDDR4, assuming the same 16 chip limit, you would also be able to configure it with up to 64GB (16x32Gb).
Gb != GB, those are 4GB full-width (x64) modules, making them 512MB per x8 chip, half the size of what Apple already uses in the MBP line's fully maxed out configuration.
Presumably they're already running it in a multi channel configuration, I don't have a modern MBP to check with but you need 8 individual DRAM chips per DIMM (or soldered on equivalent). Since the JEDEC spec for DDR3 caps each individual chip at 8Gb (1GB) they'll have 16 of them, which are probably run in dual-channel instead of dual-rank single-channel.
