I’m not sure that goose vs duck impacts the temp rating, if that’s what your asking. A 20° rated bag should insulate accordingly, regardless of the animal from which the down was sourced. Sounds like ZenBivy is taking some liberties with their ratings.
And the power-fill number refers to how many cubic inches in an ounce of down, I believe. I’ve never seen fill ratings in anything other than 50s or whole 100s, but I’ve seen them as low as 550 and as high as 1000.