Strange that this article is so current. The Dateline special ran about a year ago. Did NBC run it again as a repeat recently? To my knowledge the episode has never been available after its initial run. Unlike with most other episodes, it was never made available on the NBC/Dateline website. The network was also pretty aggressive with removing unauthorized copies on Youtube.
In fact, since this aired, there has been another Dateline story with the title "In Cold Blood", which has nothing to do with the Markel murder.
My assumption is that NBC was concerned about legal reprisal - though it never actually accused Wendi of having direct involvement. Nonetheless, Wendi's billionaire boss to come out pretty strongly against the content of the episode at the time it was released (as this article indicates).
A minor note - the above article states that Dateline is an ABC show but it's actually an NBC show. There may some confusion because, Youtube has a bunch of videos purporting to be the Dateline episode, but they actually contained a previous 20/20 special on the crime.