Not entirely true. The fact is they could have used SODIMM DDR4 which is 1.2v and has a power usage close to LPDDR3 (also 1.2v). But they would have to redesign and they would lose a very few mm in thinness.
DDR4 uses about 30% more energy in general though, hence the thickness bit as to get a 10 hour lifetime means more battery.
Presumably Apple means that lpddr is the only choice they care about for a laptop. Which while I would love more ram, understand. And that explains why if you see laptops with more than 16g of ram they generally have giant batteries and are using desktop ram.
