The headline is a bit "clickbaity," but if Mueller is demanding an oral interview, then it proves he's not interested in getting facts, but is in fact setting a perjury trap. The open-ended questions we've seen is further evidence of that.
A witness could be sharp as a tack and have expert-level grasp of all the facts, and a good Prosecutor can still trip him up. Trump needs to reject any interview, and if Mueller issues a subpoena, fire him.