Why is this writing considered a biography?

Because it tells the life of the person? I don't know what writing you are talking about.

I could not find the writing you are referring to, but I can explain what a biography is so that by reading it, you can identify why it is considered one.

A biography is the chronicle of a person's life in chronological form, highlighting his achievements, childhood events that are relevant to his life or that marked something important in a person's life, and even his failures.

Even the word biography comes from the Greek and its meaning is "to write life."

It can be narrated in the first person by the protagonist of the biography, or it can be narrated in the third person, where someone else will tell the lived facts of the protagonist.