I found it (and the rest) attributed to an unnamed physicist at another discussion board, and figured it was just a generally accepted statistic, not subject to any controversy or anything. I did look up a couple of Earthshine related studies on the Internet, but didn't find the precise figure for earthshine irradiance given in that form (W/m^2).

I have found that sunshine on a clear 25 degree day has a radiance of about 1000 W/m^2.

and I did find a paper which showed earthshine intensity at about 1/10000 of sunshine (no, wait, damn it, that's w/ respect to sunshine on the moon, not the earth, and I'm sure there's a difference due to the atmosphere)

oh well