Seems far more likely that it was someone he knew rather than a stranger. Turns out that was in fact the case.

That neighborhood isn't really known for homelessness unlike other areas of SF.

Why wouldn't a homeless guy take his wallet?

Your comment indicates you do not understand the mental state of many homeless people. I have a good friend, who has beaten by a homeless man in Los Angeles. None of his wallet, phone, or cash was taken. It was not a robbery. It was a senseless beating.