So looking at their methodology, they chose the "high class" black names ("Anderson" and "Thompson") (EDIT: I misread, these aren't the black names) based on census proportions, but that doesn't necessarily mean those names are actually read as "black" by most people.
Moreover, using census results ignores exposure to foreigners with these names. My own strongest association for "Thompson" is the white police officers in Tintin, and more generally for white English people. (Although I'm British so that may not be representative.)
As long as "low class" black names are more readily associated with blackness than "high class" ones, this methodology isn't really isolating anything, just doing a slightly more intersectional analysis.
Moreover, using census results ignores exposure to foreigners with these names. My own strongest association for "Thompson" is the white police officers in Tintin, and more generally for white English people. (Although I'm British so that may not be representative.)
As long as "low class" black names are more readily associated with blackness than "high class" ones, this methodology isn't really isolating anything, just doing a slightly more intersectional analysis.