However, the fine article says: "We admitted 10 male and 10 female weight-stable adults".
That is a ridiculously small population to start with. Yes, yes, I know it's expensive to have many participants, but part of the broader problem in today's science is that too many experiments are accepted broadly even though they are based on tiny sample sizes. This is an excellent justification for conducting a larger study, not a conclusion in itself.
> PYY (pg/mL) 25.1 ± 1.9 34.3 ± 1.9
PYY is a hormone which regulates feeding in humans. From another study:
> Several lines of evidence suggest that low circulating PYY concentrations predispose towards the development and or maintenance of obesity. 
A highly processed diet reduces the levels of PYY in the blood by around 30-40%. That's pretty damning evidence.
Would be great to see a factor breakdown (obviously hard). I can see several reasons why processed food leads to this result: chemically better tasting (optimal flavor blast + salt/falt), mentally better tasting (i.e. comfort food), or some deeper gut reasons?
because this then sets the agenda for how we fix this problem.
It's an interesting study, but I would still like to see more strict calorie for calorie, and macro for macro match.
By allowing the participants to eat freely and by having so much more sugar in the processed diet, all this really shows is that processed diet was leaving participants more hungry so they ate more.
Letting the participants eat ad libitum was a deliberate part of the study design, which was intended to develop information about whether an ultra-processed diet encourages eating more in terms of food energy than an unprocessed diet does. I'd be curious to know how you would modify the study design to achieve that end while also eliminating ad libitum intake.
This is 99.9% of the reason why people are overweight.
Every biological being eats until they're full(the body stops sending hunger signals). this is true for human, as it is ducks, monkeys, dogs or pretty much any other animal. For many, eating junk food/processed foods, breaks this satiety mechanism (and not just in the short run, there may be long term effects that prevent people from going back to normal).
Not being hungry, having satiety is the holy grail of weight management. Unfortunately, we don't have the means to produce satiety without calories. Yes, fiber/protein/micronutrients densities do help a little, but it does not help enough.
And there hasn't been enough studies on it. Other than the Holtz studies I haven't found much done on this subject.
That sounds compatible with the "Ultra-Processed Diets Cause Excess Calorie Intake and Weight Gain" conclusion in my opinion.
Eating whole foods is healthier than eating packaged foods is pretty intuitive at this point.
In a study that did not allow free eating, we could have gone further to find out if a 100% controlled ultra processed diet, was still worse for you the subject that a calorie matched diet based on whole foods.
It's absolutely relevant, considering the conclusion of the study was that people feel less satiated after eating a diet high in processed food, thus continue to eat and consume more calories.