The special counsel abdicated on obstruction to avoid a confrontation with the Justice Department and get his evidence to Congress.
Why mention the OLC guidance at all?

That is the question for Bob Mueller, left hanging by the statement his office jointly issued with Justice Department flacks on Wednesday, clarifying (as it were) remarks he had made hours earlier at his parting-shot press conference.

At issue is Mueller’s decision to punt on the question of whether President Trump should be indicted for obstruction of justice. In his startling remarks, Mueller sought to justify himself by citing instruction from the Justice Department’s Office of Legal Counsel.