I don't like the study setup there. One readthrough of spoiled vs one readthrough of unspoiled material lets you compare the participants' hedonic ratings of dramatic irony vs mystery, and it's quite reasonable that the former would be equally or more enjoyable... but unlike in the study, in real life unspoiled material can be read twice: the first time for the mystery, then the second time for the dramatic irony; with spoiled material you only get the latter.