It's not a good proxy when the data is so messy. In your example, is there actually discrimination or are black people just less likely to be able to afford the homes that this realtor is selling? Are black families just less likely to want to own homes?
Archie makes the argument that this is special pleading: yes there will be cases where the underrepresentation is not caused by discrimination, but generally discrimination will be the, or at least a cause. I just don't think the world is that simple, and don't think that from underrepresentation we can assume discrimination.
I think that it's entirely possible to make a good case based on statistics, but not naively.