The suggestion in that article was that it may have been related to the protein content: "The ultraprocessed meals contained slightly less protein, and some research has found that people tend to eat until they reach a certain protein target". Although I'd have thought the sheer bulk of indigestible fibre was more likely to affect participants' perceptions.

The 'processed' diet included "fiber-enriched beverages" in order to compensate for the comparative lack of fibre in its content. Apparently the follow-up study is going to further increase the protein levels and try using soups instead of soluble fibre supplements in an attempt to narrow down the findings.