In a way, and in some sense, I do agree with you that one’s acquaintance with a culture is useful in writing about that culture. It is not sufficient, of course (think of the nonsense written by most anthropologists during the last 100 years on other cultures); nor are there any compelling reasons to believe that it is a sine qua non to understanding cultures. However, I am not challenging the claim that the context of the person might be relevant to producing knowledge.
Of course, there is a difference between being born in some culture and trying to understand the same while coming from another. But that does not mean that one cannot understand a culture unless one is born into it. It is like saying that the only way to understand ’neurosis’ is by being a ’neurotic’ oneself. Obviously, I cannot buy it: why can I not understand Christianity without being a Christian? If I cannot, even a process such as religious conversion would be impossible. History would be impossible too, as well as the possibility of social sciences. One needs to give extremely good reasons why one cannot understand a culture without being born in it.