Everything you say makes sense, but all of those points seem to require at least a bit of reaching. If anything, it appears to me that the depiction of the First Slayer was a general recognition that humanity (almost certainly, according to the best evidence currently available) originated in Africa; further, the intention seems to be that the Shadow Men be seen as the self-appointed controllers that eventually became what we call the Watchers, so that any adversarial content contained is gender-based — “the patriarchy” — rather than racial.

It’s one thing to speculate on the possible implications in a story, and all your speculations are valid. To simply call it racist, however, makes it sound as if the writers have been found guilty of any attitude you are possibly able to imagine in their work. That seems a bit stern.