It was not a third hand story (US intel official claimed direct knowledge),
  reporters are not second hand (they interpret, contextualize, and synthesize)
  and editors are not third party (they are an integral part of the news
  process).

It literally, according to the definition of third-hand, was a third-hand story. This isn't up for debate, as the term can be found in any dictionary.

Your parenthetical on the direct knowledge is wrong. Direct knowledge was not stated anywhere. That is your spin or second-hand version of what was written in the slack thread, which was at least second hand derivative of the missile strikes.

  There was an editorial failure, certainly, but in a rapidly changing situation
  due to a misunderstanding of the evidence given by a vetted source (such
  sources are seldom named even internally). Don't forget five or six nations
  were "squabbling" over the origin and nature of these missiles for days
  afterwards. Russian missiles did hit Poland, but they were Russian-made, not
  Russian-launched. It's not hard to imagine how easily this message could be
  muddled somewhere between Lviv and New York.

So, you do understand why the story should not have been written. You just choose to side with people continuing to write stories in that manner. There wasn't a misunderstanding of evidence. There was a complete lack of evidence, yet a story was published.