Because in the partial transcript of Matt Cutts audio, Matt didn't mention that Panda would be integrated into the search algorithm.
Here is a partial transcript of the audio clip with Matt Cutts which was refer to in the said article:
Rather than having some huge change that happens on a given day. You are more likely in the future to see Panda deployed gradually as we rebuild the index. So you are less likely to see these large scale sorts of changes.
Furthermore, Barry posted on 15th March 2013:
We were told from a Google spokesperson.
"I don't expect us to tweet about or confirm current or future Panda updates because they'll be incorporated into our indexing process and thus be more gradual." [searchengineland.com...]