Furthermore, libavcodec is not a DivX codec. It's a codec library containing many audio/video codecs. The author probably meant its MPEG-4 codec.
And DivX5 is not included in MPlayer. Again, the author meant the MPEG-4 codec from libavcodec. This is a common confusion - DivX is not a format, it's a commercial MPEG-4 codec made by DivX Networks. There are other MPEG-4 codecs, some of them open-source (libavcodec, XviD).