It brings the “focus” on one single white person, fine. But this is petty complaining — they’re attempting to make a black person feel better by letting them know that they’re aware of what “white people” have done and they personally feel sorry for it. It’s logically moronic of them to do so — one white person is not responsible for “white people”’s crimes against minorities — but they are doing something good. You could say that those who head donations are “bringing the focus on themselves”. Or those that protest a business that practices segregation are “making it about them”.
And it’s also an assumption that a person who would do something like this would expect you to “manage their white guilt”, maybe they’d appreciate a black person who wasn’t feeling as incredibly oppressed as you are to take some of their free time to discuss things with them, but where are they expecting anything? I’m sure they understand not every black person would agree with them or even has time to “reply”.
How does one even “reply” to someone standing with a sign, anyway? It’s not like they’re sending a message on twitter to all black people with “I’m really sorry”.