You misunderstood the comment you replied to. The question mark was meant to indicate a stipulation of someone else's point.
The person you replied to wasn't saying that library fines affect poor people disproportionately, but rather saying "Hypothetically, if we stipulate for the purposes of discussion the previously claimed statement that library fines affect poor people disproportionately, then ..."
Any poor people who are fine, upstanding, responsible, on-time returners of library books are out of scope for this discussion. The person you replied to was only talking about the hypothetical, maybe don't exist at all, poor people who don't return their library books on time and therefore suffer financial harm from fines.
I practically lived at the library when I was a kid and I didn't have much money to spare, so I was very, very aware of due dates and didn't take chances. I had very, very few incidents of returning books late. I have one traumatic memory burned into my brain of the time I realized that I had set a library book down on the top of my mom's car and not picked it up before we drove away. (It was Riverworld by Philip Jose Farmer and nothing like that ever happened again. Losing a library book and having to replace it was an experience that taught me a lasting lesson.)
It's the people claiming that library fines disproportionately affect poor people who are casting moral aspersions, not the person you replied to. It's those people, not the person you replied to, who are claiming, accurately or inaccurately, that being poor somehow renders a person less capable of comprehending and honoring library book due dates.