Well, I'm going to be sheepish again. It fixes some gaps in the drafting.
The Auditor General derives powers, duties, and functions from two places. As you'll note, in Bill C-2 the proposed subsection 18.2(1), the last line--the easiest reference to find—says, “powers, duties or functions under this Act”. That's a reference to the Auditor General Act. The Auditor General also obtains powers, duties, and functions to conduct audits under part 10 of the Financial Administration Act, which pertains to crown corporations; thus the amendment suggests referring to “powers, duties or functions under this or any other Act of Parliament”.
So the amendments proposed in G-58 are simply to bring the bill in line with where she actually derives her sources of authorities from.