Comparing the study cited to my local conditions in Hawaii, I see the study was done in Cassilāndia, which is about 19° from equator, about the same as here, and the temperatures measured during the study were mostly around 26°C, also about the same as here. In other words, a mild climate that rarely gets hot. So the study seems to offer no insight on the above questions of extreme heat tolerance.
The question remains how the sun tolerance changes with age of plant, since the study tested tiny plants up to 201 days of age, and all the anecdotal evidence suggests achacha (and mangosteen and other Garcinia) adapt well to full sun at some point. My friend who has gotten 2 generations of fruiting achacha here has grown them in full sun their entire lives, with fruit after 6 or 7 years from seed. He often talks about his belief that they like full sun, but it seems based on speculation. Maybe we can do a little better with more shade, but at least it's not a critical issue in practice.
Perhaps as the plant ages, its lower canopy gets shade from its upper canopy?