Long term historical trend, lots of small tech improvements that add up, like all other tech. Some of it's how antennas are higher gain, which puts more of the energy in the path from one end of a line to the other and wasting less (affecting both cellular and WiFi standards over this period), some is improved compute reducing the cost of routing, but as with the improvements to chips and batteries and PV, the list of things is long and each one only contributes part of it.

EDIT: got the maths very wrong with some other estimates, deleted them.

Seems reasonable to believe to me. The cost of a transfer is presumably calculated based on the base power cost of the transfer machinery, since I really doubt that a router or switch's power usage is linear with the amount of data it's transferring. The amount that an industrial router or switch (which is what 80-90% of the hops between you and Google are) has to have increased its bandwidth by around 10x over that time, and I doubt they have 10x'd their energy usage.