Actually, the author (Jamil Zaki) goes a lot farther than that:
Psychological studies are not about you. They make few if any predictions about how you should live your life, how to tell if you’re an introvert, or anything else about you as an individual.
So, it’s not that it’s uncertain if such studies apply to you, it’s that they make “few if any predictions” about you.
The justification for this assertion is problematic, though. Zaki writes:
A typical study might include 200 people, dividing them into groups (say, people told to act generously versus those told to act selfishly), and demonstrate a statistically significant edge in happiness for one over the other. Like a batting average, though, even strong differences across groups tell us virtually nothing about how generosity or selfishness would affect the happiness of any one person….
…psychological studies, without telling us about any one person, can tell us about how changes in behavior (again, think generosity) might affect the well-being of whole populations.
A strong difference across groups is, in the vast majority of cases, a difference in the means of some measured variable. Depending on how the individuals making up the groups are distributed, a group (mean) difference can tell us quite a bit about how generosity or selfishness (or whatever experimental manipulation we’re interested in) affects the happiness of any one person.
Of course, an experimenter has already observed how their manipulation affected the individual people in their study. And if we’ve got a reasonable model, we could, in principle, generate (probabilistic) predictions about more or less likely effects on a thus far unobserved individual. The predictions will be noisy, and they will be uncertain, but it seems too strong to say that measured group differences tell us “virtually nothing” about individuals.
If changes in behavior affect whole populations, then by necessity, they affect the individual members of those populations.