I don't know the historical background of this letter, so what follows is purely a close reading of the text itself.

I've studied style. The sentence structure here--especially in the first section--is definitely 20th century. Short sentences, simple syntax, very little suspension (e.g. there is little starting with a dependent clause; it's simple subject-verb-object word order, with only a single clause or at most two).

In fact, each of the bold-faced sections is a little different. Those that follow the first one are more complex syntactically (but still with plenty of tells that this is a mid-20th century or later construction). I wonder if several people may have created this letter.

In general, the punctuation, phrasing, and idioms would indicate that many or all of the writers are college-educated but not academics (and not English majors--perhaps history of political science).

Rhetorically (and this is not based on style, syntax, or punctuation), the letter is most likely ironic, i.e. intended not to promote slavery but to alert Blacks in the post Jim Crow era (i.e. after 1960 or so) to the underlying systemic oppression of Black Americans, e.g. the reference to skin shade--which is not likely to be a major concern for an enslaver, except insofar as white racist enslavers might prefer so-called light-skinned women as sexual objects. Modern Afro-centric thinkers are more likely to be sensitive to this distinction (hence the popularity of the phrase "Black is beautiful" in the 60's, or Malcolm X's reference to coffee with cream). Rhetorically, I believe, this letter is intended as a kind of wake-up call to Blacks in the 1960's-1970's. In his early speeches, Malcolm X (who is woefully underestimated as a rhetorician) made a number of wake-up calls to Blacks during this period but without the sort of irony or fictionalization that's evident in this letter.

So in sum (and again, with no external evidence), this letter seems to be a collaborative effort by politically aware Black thinkers, probably from 1960-1980. The effectiveness of the effort is evident from the variety of people you refer to who have quoted the letter.