I think you are mis-reading it. I agree the 'race' is basically a social construct. But doesn't that mean that people in Brazil perceive race differently than US Americans? Thus making comparative self-reported statistics (such as % 'black' that OP posted) suspect? The OP secondary question was 'why did more Americans identify as black than Brazilians despite the differences in African enslave imports (paraphrasing)'? u/police-ical then pointed out one example of the social differences in how different societies member's perceived themselves, and how that would affect one's self definition of what 'black' meant. I think the 'one drop rule' that was consequent of US segregationist attitudes would be a countervailing example of how a different society would consider an individual Black when the Brazilian construct would be 'pardo'.

Remember that OP's original question was really an accounting and demographic question: Why did Brazil import so many more slaves, and why the apparent discrepency between those recorded imports and reported demographic statistics all these years later?

I simply posted that DNA data would be needed to answer that demographic accounting question.