The parent reply here is arguably just "priming" the reader to remember that donations are an appropriate way of showing thanks for the help. It's not that people are unwilling or unable to afford to give a donation (although some cannot), its more that they don't even consider it in the first place. A gentle reminder, even smaller and simpler than the above example, just prompts them to consider if they would donate.
I was going to say that I think people kind of build up an immunity or aversion to those sorts of requests as they become more common, but after some consideration there's a difference between individual vs mass communication in that regard.
