The title of the thing says "editorial" and it is signed and dated as a letter by somebody who works for the magazine. How is that NOT an editorial letter?
If you had read it, you would know that it's not dated. It might be splitting hairs to note it's not written "as a letter," if by that you mean in a form common to letters exchanged between correspondents, from readers to editors, or from editors to readers in response.
However, it is from the desk of the president of the journal's governing board -- a journal of African history and culture, I'll remind you. So, if an expert on African history presents this 200,000 figure as uncontroversial, what reason do you have to doubt it?