IMHO - The best pellet, is the one you bird will eat some of. My Grey and Conure will eat small quantities of Harrisons Coarse High Potency and Harrisons Spicy. They will not touch any other pellet.
I ensure they get all the vitamins and minerals they need from veggies, nuts, seeds, eggs, Red Palm Oil,a little fruit and direct sunlight and - or UV lighting.
I have a feeling a study as indicated, was based on some birds being fed a "Pellet Only" diet, which is not healthy at all for any Parrot, bird or other critter for that matter.
It would be interesting, if the study is available for reading on the Internet somewhere. Is it posted? The way it was performed, what species Parrot was used and for how long.