(BTW, "if I were..." is correct.)
That is not at all accurate. It would depend on the tonality of the background and the size and position of the white bird in the frame. And you have forgotten that there was a big black gator head in the middle of the image... If you photographed a bright white bird in the same light, however, you would likely be fine with by setting either 1/4000 or 1/3200 sec. at f/5.6 manually.
As I said, those who wish to learn exposure theory need to go back to the original "The Art of Bird Photography: and study the great chapter on exposure. Otherwise, they are doomed to fail when an amazxing situation with a challenging exposure takes place right in front of them. BTW, you are in good company; several very talented photographers flunked the quiz.