Mueller didn't just wait to call Buzzfeed out, the special counsel was specifically contacted by Buzzfeed and declined to comment on the story. That means the special counsel knew it was at least being looked into and specifically decided to give Buzzfeed enough rope to hang itself. Maybe they didn't think anything would come from the report, or maybe they wanted to let the media work itself into a frenzy only to pop their bubble after all the fireworks had been blown. Only time will tell for sure.
>A spokesperson for the Office of Special Counsel declined to comment.
https://archive.fo/ETWuT#selection-1189.0-1189.69