This seems ridiculous on its face but if it gets more people to prioritize efficient, performant code, then I'm all for it.
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.
It's like telling people to take shorter showers in a drought, when residential water consumption is a miniscule fraction compared to agricultural or industrial use. To the point where short showers affects nothing and benefits no one. When we really should be cutting allotments given to farmers and industry. This is something that goes on very frequently where I live.