I don't think this is accurate. The joke to me is not that he "can't play" but instead he can play, just very badly. In Psirens, he certainly plays guitar better than I can (as terrible as it is). In Marooned you hear him play about 3 chords (though it is implied he actually plays an entire song or two). Both times we've seen him play guitar (are there any others?) he's been playing it with his own guitar which is quite obviously not tuned correctly. I don't see him playing guitar in this episode anywhere near a continuity error. It can be explained by the better parts of the playing being mostly Hitler, or Lister playing better because he's playing with someone else, or him using a guitar which is tuned properly. Either way, the performance with Hitler was still crap.