Point taken, I'm probably using "round/flat" as shorthand for a lot of other things, and yes, the underlying issue is humanity. On the other hand, "this character feels human" is a subjective end-effect, and the direction "make them fully human" doesn't give you much specificity as to how. Lack of agency, goals, interests, conflict, their own agenda, is objectively easier to determine by looking at the words on the page, so these things serve as helpful guidelines when in doubt. Is it possible to depict a character whose own goals, interests, emotions, and agenda are invisible or opaque to the reader, and who nonetheless reads as fully human? I expect it is. I expect it would be a result of masterful line-level writing? I'm not coming up with an immediate example, though.
Not every minor character who doesn't merit full description is going to read as an example of illegitimate exploitation. If the butler who announces the detective and gets one line in the whole book does not have his own visible-to-the-reader life struggle, I don't think it's going to grate on anyone. But your typical Magical Negro is much more foregrounded than that. They are salient; the Message they convey is part of the book's Message; their trust in the Hero solidifies our trust in him, and their unjust demise at the hands of the cruel Baddie is calculated to arouse our outrage. If the character is bearing that much weight in the book, then the fact that their own agency, struggles, subjectivity, goals, etc. are unknown to us -- or treated perfunctorily -- is noteworthy, and will tend to dehumanize them.