Sounds like a good bet. Also a good way for the author to push a lie without actually lying (I'm sure most of the readers would have equated 'religious' with meaning 'Christian'), and the author would do nothing to deter that notion.
Fwiw, the study said results for Christians and Moslems were similar. It would be interesting to see the correlation of other factors, such as income. Maybe children from better-off families are more inclined to give away hypothetical stickers to an anonymous, same-ethnic-group peer in their school.
Or maybe religious children had more of a “This study is crap” attitude toward the study, “Just give me the stickers so I can leave.”