The input is the claim 'Race is a cultural convention.' You output the interpretation: 'None of the phenotypic variations associated with any racial schema are physically real; they are hallucinations or figments.' Given how transparently ridiculous the assertion is, one must at least take a moment to pause and reconsider whether the anthropologists' claim is really what you take it to be.

The problem is that when asked to justify that statement 'Race is a cultural convention' anthropologists in interpret it in the way you describe in your second paragraph, but they than proceed to use it in arguments as if it means 'None of the phenotypic variations (except possibly skin color) associated with any racial schema are physically real; they are hallucinations or figments'.