Also from the article:
“I feel like a lot of people who do a vegan diet think ‘Oh great, soda is vegan. Pancakes are vegan.’ No, they are refined ultraprocessed grains,” Gardner said. “So, we tried to get a healthy vegan diet and show them quickly what that was for four weeks.”
The meat and veggie group also had meals delivered that were a step up from their normal fare.
Sounds like the guy is a health food nazi, and you have to wonder exactly what he was feeding them that they'd eat less just because they're sick of it or whatever. Like yes if some pancakes are made with no animal products, that's vegan. Sounds like he did the same with the 'meat & veges' meals, but even a health food version of that is going to be more like what regular people are used to eating.
Vegan food can be plenty tasteful, and it can also be junk food either as a specifically vegan product or just regular non-dairy junk food like a bag of chips. Really the biggest challenge with the North American diet is that so much of the meal has meat as the center piece that if you simply remove it you are now lacking in taste and nutrition.
Like say you have a chicken caesar salad at a restaurant, and for vegan they simply replace the chicken with an avocado, that's no longer a balanced meal. You kind of have to rethink the whole process to make it work vegan, but it's doable. In this case one of my go to summer salads is a caeser that uses cashews, chickpeas, hemp seeds, and nutritional yeast, that's a perfectly balanced meal and I'd put it up against anything you could get a restaurant meat or otherwise.