First, the study was conducted in a controlled environment using self-contained mystery/twist-based short stories that the subjects probably never intended to read otherwise and to which they consented to being spoiled on in advance as part of the study. The study also focused on
the role of suspense in suspense-based stories. This is a very different situation from a person watching a beloved action serial and being spoiled on the events of the ending before they had a chance to watch it.
Second, even the people who conducted the experiment admitted that it's not black and white; note that satisfaction went down when the spoiler was inserted into the story instead of being told in a paragraph in advance. Why is this? We have no way of knowing from the information we're given (at least, it's not described in the press release), but at the very least it's clear that there's more to the science of spoilage than just "spoilers good, stop complaining."
For that, it would probably help if you'd link to the
actual press release where the purpose, methodology, and researchers' impressions of the study are discussed in more detail, instead of editorialized versions of the study posted on media sites cherry-picking quotes for the sake of smarmily informing their audience that science says they're wrong for not wanting spoilers.