Yes, but.
I thought that, but it does specifically say that they've done it for email and most of such print on one page (or more usually the top 10 or fewer lines of one page) in any font at around the 10/12 point size. So this argument's moot, bar for the trivial case of the odd very log one that happens to be exactly the right length to throw a second page when printed in CG that would not have been thrown in Arial.
I'm more interested in how they (or whoever) found this. Presumably someone, somewhere printed off the same several thousand pages of text in umpteen different fonts, carefully measuring the ink use for each run. I guess that to save the planet you have to kill it a bit first....
I'm also a shade ticked off that the winner didn't turn out to be Comic Sans. The indignant flamefest around here at the serendipitous confluence of two pet hates prevelant amongst the assembled El Reg commentards (ecofiddling and Comic Sans) would have been a joy to behold.