The argument doesn’t claim that aspirin prevents heart disease, just that it improves health. The flaw lies in the presumption that this applies to most people in these nations.

This describes how the argument fails to justify the claim that aspirin would benefit the majority of people. Simply establishing that the disease it treats is common isn’t enough.

This is irrelevant. The author's claim is that aspirin helps with heart disease, thereby improving health. The author assumes that this will help most people's health because heart disease is one of the most common illnesses in these countries. The author doesn't have to assume that helping with heart disease must have some effect on other diseases, too.

This is irrelevant because the doctor never claims that it’s the best at preventing the illness, only that it helps. The flaw is assuming that this would be helpful for most people in these countries.

The conclusion is limited to people in industrialized nations, so it doesn't matter whether the studies were conducted only in industrialized nations.