MUELLER: “[W]e concluded that Congress has the authority to prohibit a president’s corrupt use of his authority in order to protect the integrity of the administration of justice."
It's not explained why Mueller made no finding here
But Mueller had already determined he wouldn't "draw ultimate conclusions" because of OLC restriction.
-Mueller set out not to make a decision on obstruction because he couldn't formally charge and Trump would have no legal recourse to clear his name.
-But he demonstrated multiple episodes in which Trump met the criteria for obstruction. Said Congress is ultimate arbiter.