While certainly not the largest losses, they do not appear insignificant. In LPDDDR4 they introduced[1] a new low-voltage signalling, which I doubt they could have gotten working with SODIMMs due to the extra parasitics.

If you look at this[2] presentation you can see that at 3200MHz a DDR4 SODIMM would consume around 2 x 16 x 4 x 6.5mW x 3.2GHz = 2.6W for signalling going full tilt. Thanks to the new signalling LPDDR4 reduces this by 40% to around 1.6W.

Compare that to a low-power CPU having a TDP of 10W or less a full 1W reduction per SODIMM just due to signalling isn't insignificant.

To further put it into perspective, the recent Lenovo ThinkPad X1[3] uses around 4.15W average during normal usage, and that includes the screen.

Obviously the memory isn't going full tilt at normal load, but say average 0.25W x 2 sticks would reduce the X1's battery lifetime by 10%.

edit: yes I'm aware the presentation is about LPDDR4 yet the X1 uses LPDDR5, just trying add context using available sources.

[1]: https://www.jedec.org/news/pressreleases/jedec-releases-lpdd...

[2]: https://www.jedec.org/sites/default/files/JY_Choi_Mobile_For...

[3]: https://www.tomshardware.com/reviews/lenovo-thinkpad-x1-carb...