> This seems ridiculous on its face

Chip designs routinely use energy metrics like picojoules per bit to express the hardware's power budgets and efficiencies, generalizing these kinds of figure of merit to software seems natural. While I doubt that running a particular version of software can automagically make a computer "green" (If WebKit becomes faster, users are simply going to open more tabs), but I think this can be a reasonable and meaningful concept within a clear technical context.