The article goes through this. The claim is that RStudio knowingly pushed false information about the incident even after DaraCamp reached out to them to correct factual inaccuracies and false claims clears up by the independent third party investigation.
Reporting on an internal incident isn’t defamation in and of itself, but if you knowingly sprees lies about the incident to damage the reputation of a competitor, then it is.
The article states that the initial "independent third party investigation" was not all that independent. The investigation was done by an investor who had put a substantial amount of money into DataCamp.
The second was carried out by a contractor as a response to the initial outcry. This has been sometimes referred to as an "investigation", but its authors frame it as a "review" targeted at helping DataCamp correct workplace issues. You can read the report here: https://www.datacamp.com/community/blog/working-ideal-indepe....
As far as I can tell, neither of these processes concluded anything that contradicted the BuzzFeed article in any significant way. The contractor's report concludes "there is little factual dispute about what happened". Since DataCamp's most recent post isn't specific about what factual disputes exist now, I guess we're in the dark about that.
Reporting on an internal incident isn’t defamation in and of itself, but if you knowingly sprees lies about the incident to damage the reputation of a competitor, then it is.