Makes sense. A place with racist clauses would attract racist people. I mean, racist would see "only white" as something good and would pay more to live there because racism. Those people would pass their (racist) values to their children. Non racist people would feel uncomfortable around racist people, so non racist people would choose other places to live.

Even if the clause is illegal now, if a normal person sees it now, it is warning of the kind of neighbours you might find. If you have other options, it makes perfect sense that you would avoid having racist neighbours.

What you have is a self selected sample, not a random sample of white people.

On the other hand, you would be more likely to find non racist people living in the same neighbourhood because a racist person would feel uncomfortable living around non racist people (also know as "woke snowflakes") because they would feel that they "can't say anything" (also know as people reacting to their racism).

I found your article really interesting because it shows how certain ideas get perpetuated.