No surprises here. It is indeed extremely difficult to hear such small differences (for most mere mortals)
I did extensive test with Sony's Atrac compression and concluded I could hear a difference but it was not at all easy. The same result for iTunes files AAC 128 Kbps - often it is extremely hard to tell them against uncompressed files but generally you can with careful & extensive listening. Hoever, somewhere above 256 Kbps compressed files the ability to tell the difference becomes exponentially harder...just my experience with my tin ears....so I am hardly surprised by the findings in this article.
