Three circumstances in the case study /essay tell us that Dorio sees it as wholly applicable to the general populace. The first circumstance is the situation in which his encounter with Ryan occurs. The second is his manner of addressing his audience in the case study. The third is the end result of their second meeting.
1. When Ryan and Dorio first encounter each other, the situation is that Dorio is at the gym as a private human being, not as a medical doctor, who is doing his own physical fitness workout in his private, individual capacity. Ryan is wheeled in a wheelchair into the gym by his caregiver to do his physical therapy training; this isn't in the same capacity as Dorio's workout but rather it is dedicated work to regain body function:
[Ryan] wasn't there for the [macho] image, though. He was courageously attempting to bring his body back from a life-changing catastrophe.
When they encounter each other, when Dorio sees Ryan training with "futile attempts — and frustration," they both are members of the general population. It is only later that Dorio meets Ryan a second time in his capacity as a medical doctor. This capacity, as members of the general population, extends the relevance of the encounter to general population.
2. Dorio addresses his audience through two modes of acknowledgement. First, he addresses his readers as colleagues: "As physicians, we have the privilege...." Then, he addresses as fellow private human beings: "As human beings, we are also given the ability...." By doing this, Dorio extends the case study to the unlimited bounds of applicability to the general population: he removes the case study from the confines of medical expertise and opens it to consideration by all human beings: the general population.
3. The end result of Dorio's two initial encounters with Ryan is that they build a mutual human friendship in which they mutually impact each other's lives. This mutuality can only exist when both present themselves as equal human beings, thus, as representatives of all human beings in the general population, extending the applicability of their experience to the general population. To illustrate this, Dorio tells us that, now, when Ryan sees Dorio at the gym, he is inspired to work even harder than before, while when Dorio sees Ryan, Dorio is similarly inspired to work harder himself.
Now, when Ryan sees me at the gym, I see him work harder. And when I see Ryan, I work harder, too.