We are not actually explicitly told why Jeffrey does not have any fear of the Finsterwald house in the way that all of the other kids in the neighborhood do. Spinelli does not come out and say anything like “Jeffrey (or Maniac) wasn’t afraid into the backyard because…” Therefore, we can only infer the reason. The reason has to do with Maniac’s character. He is a person who is not inclined to be like everyone else. This is one reason that he, for example, is so comfortable as a white person living among black people in some parts of the book.
We see from early on in the book that Maniac is not afraid of the Finsterwalds’ like so many of the other kids are. In Chapter 5, he casually goes into the back yard and rescues Arnold Jones when high school kids throw Arnold into the backyard. Much later, in Chapter 36, he goes into the backyard and even knocks on their door as a way to get the McNab boys to go to school.
Superstitions, like prejudices, are just silly ideas that people have come to accept. If a person does not accept them, they have no force. That was the case with Maniac. He did not buy into the prejudices and superstitions that others had.