Some people undergo poverty, illness, or hardship while others enjoy ease, health, and prosperity. If the most pleasant or beneficial state in every respect were necessarily due to each person, such variation calls for explanation.
The source uses the example of an impoverished unbeliever to test claims about obligatory benefit across worldly and ultimate outcomes. It does not authorize declaring the final fate of any particular identifiable person.
Benefit is not necessarily identical with immediate comfort. Yet broadening its meaning does not, by itself, demonstrate that granting the optimal benefit is an obligation upon Allah.