If the following is true, then MP3’s are always decoded/converted to PCM, whether it’s Wavelab or iTunes or Sonnox or Windows Media Playber or whatever player/editor. The MP3 files are never played directly because the hardware doesn’t support it. The hardware supports PCM. But Wavelab and most DAW just convert the whole file at once, rather than in pieces. That would be the only difference I think.
I guess there are some editors that edit MP3 directlly, but I would think they still convert to PCM in order to hear the result.