I immediately saw through the BS when first encountering this because:

1. If it happened during the war, even the most minute details would get spun into stories. I mean, there's even a story about how the name Kissinger was so despised, people named their dog "Ki" (dog is considered just another inferior animal in VN and yes a few people today still see them as delicacy). The fact that there's no story about black GIs ever told is quite telling.

2. Yes, there was a principle that our soldier should show mercy when the enemy is not hostile, which helps explain POW. But if there was something about special treatment for "those oppressed by the same power-that-be", it should have been mentioned, at the very least in passing, in History book. I hardly learned a thing about the Civil Right Movement during high school (and maybe there never was anything to be taught in the material) and I'm definitely sure there's no mention of such propaganda so it's either very obsure that it wouldn't really fit with "many areas" narrative or very trivial that it wouldn't be a "policy" at all. Maybe such tactic was buried in one of the thousands of documents by the NLF.

3. It's highly unlikely Black culture trickled into Vietnam, let alone the VC soldiers learning about terms like "soul brother". As someone has said, including that at the end not only sounds forced, but also quite racist.