FFmpeg's AAC encoder is not finished (yet?), and flagged as experimental. Including it in such a test is rather a dubious idea: it is likely to give a bad impression of the whole project.
Having the new vo-aacenc as contender for the Free Software community would IMHO have been more relevant.
Well they ARE using it as the low anchor, i.e. they know full well it's gonna sound bad so if people wind up rating it higher than any of the other options that's an indication that something odd and/or wrong has happened, plus it's helpful just to see a worst case scenario option on the list.