He’s not refusing to testify, but simply insisting that any congressional testimony he would provide would not go beyond his report. “We chose those words carefully, and the work speaks for itself, and the report is my testimony.”
Mueller sought to explain his thinking more fully. As an employee of the DOJ, he was bound by their guidelines. Therefore, he said a president “cannot be charged with a federal crime while he is in office. That is unconstitutional.” And he noted, “Even if the charge is kept under seal and hidden from public view, that, too, is prohibited. Charging the president with a crime was therefore not an option we could consider.” The Constitution “requires a process other than the criminal justice system to formally accuse the president of wrongdoing.”
However …
“If we had had confidence that the president clearly did not commit a crime, we would have said so.”
In other words… “we, the jury have decided the defendant is not innocent.”
Mueller is being just a bit too cute.