Why it is harmful? The only answer I have found in the article, except of observed data, is that UPF is "not natural". Being low in protein can not be considered as harmful food despite that eating that food constantly is unhealthy nutrition.
Dr. Greger has a good video that lists lots of other problems with ultra-processed foods: Ultra-Processed Junk Food Put to the Test: What happened when ultra-processed foods were matched for calories, sugar, fat, and fiber content in the first randomized controlled trial?
https://nutritionfacts.org/video/ultra-processed-junk-food-p...
Video is 7 minutes. Transcript and list of sources cited are at that page, too.