You can eat junk food filled with carbs (Standard white bread, pure pasta, white rice), likewise junk food full of fats. (Most milk products, highly processed meat.)
The main problem is when the diet is either lacking in things we do not completely understand (vitamins, micronutrients, perhaps phytonutrients) or is effective caloric surplus - that is, you gain weight.
Considering food x as carbs or y as fats or z as protein is reductive to the extreme not warranted by current state of knowledge and leads to milk powder and mashed potato diet. ;)
The basic nutrients do not have a quality other than perhaps the ease with which they're absorbed, which early matters for even somewhat unhealthy person, only perhaps for really sick people.
Our knowledge of more than coarse grained dietary needs and effects is grossly overestimated. That vacuum is exploited by commercial interests that typically vilify any form of consumption moderation, and focus instead on promoting one (over)consumption pattern over another.
"Eat lots of different things in moderation because honestly we do not know in enough detail what we need" is sound advice, but sadly just benefits the "patient" and not some profit maximizing economic lobby group.
As for other meats and dairy(which is just milk), most serious nutritional researchers will tell you to limit those if not nearly eliminate them from your diet. The media is constantly showing pro-meat stuff because the meat and dairy industry keeps funding pro meat research. It's especially confusing because even some of the largest companies/organization like the AHA and American cancer society is taking money from big meat and big dairy and funding pro dairy/meat research.
A McDonald's hamburger is a good example of "meat" that would be good to limit.
I think the point is the "state of mind" is a product of everything else going on in the body. It will be infinitely more difficult to have a positive state of mind while your body is contending with chronic inflammation and your unhealthy gut isn't producing serotonin. Your right 3 weeks isn't enough, but biomarkers can be dramatically improved in 3 weeks with change of diet, especially with respect to chronic inflammation.
They don't seem to have done the 3-month follow up with the control group.
Most importantly it is not a cross design, and even that would need in excess of hundred patients to be meaningful. As is they can be measuring the effect of attention and any kind of change on depression, given some genetic, phenotypical or environmental susceptibilities incidentally found in their treatment group.
Meaning the conclusion is not possible to generalize.
These tiny studies, especially of low quality and poor design are an annoying waste of time and money.