I guess the study used the modifier "wealthy" along with developed to explain their choice of reference class. I looked at the list and it didn't seem obviously cherry picked. What countries would you add?
The proper reference class probably depends on what this is being used as a proxy for: the ratio of self-described health to medical quality-of-life metrics is an odd enough figure that I assume it's being used as a proxy for something. If we're looking for degree of overconfidence in health care efficacy, which seems like the most likely candidate, using the first N countries ranked by per-capita health care spending might be the way to go: that gives you a list that's not too dissimilar from the one in the article_per_capita), although some of the detai...
Of course, it turns out they're actually last among developed nations in real health outcomes.