My somewhat educated take on that is that 'perceptually lossless' refers to quality at which the source cannot be reliably picked from the source/encode in any sort of test that would pass as scientific, possibly outside of a few very rare edge cases (sound samples, not people). This happens at lower bitrates than many people might imagine, IIRC v2 lame (~192kbps) has not been reliably identified, nor vorbis v6 (~160kbps). So their claim of 'perceptually lossless' at 256kbps is neither surprising or impressive. (it may be competitive/better at those high bitrates and they are just being conservative with the claims).
Usually it's double blinded ABX testing: A computer program gives you: Encoding A, encoding B, unknown encoding A or B. You have to choose if X is A, or X is B.
One of the encodings will be "uncompressed" when testing if the format in question is perceptually lossless.
Repeat that a couple hundred times with many different listeners using standardized samples and programs (doom9, an audiophile forum, does such runs every now and then), and you get a rather good idea on what's going on.
As for the 192kbps: It also depends on the algorithms used. bladeenc or 8Hz-mp3 back then created 320kbps files where you can easily hear the difference. Current lame builds at 192kbps? not so much.
Worth noting it is 'v2', which is a specific setting with a specific encoder (lame) that creates a variable bitrate file which averages around 192kbps.
That is very different to just setting itunes to 192 and expecting 'perceptually lossless'.
