I suspect the definition is worth making because even if we don’t know what caused the bias, we can use the label of a bias “not inherent in our mental machinery” as a marker for study of what it’s cause is in the future.
For example, I read in a contemporary undergraduate social psychology textbook that experimental results found that a common bias affected subjects from Western cultures more strongly than it affected subjects from more interdependent cultures such as China and Japan.
[Obviously, my example is useless. I just don’t have access to that book at the current moment. I will update this comment with more detail when I’m able.]