His interpretation is how it should be read in English. It may be not how it should have been written.

> 78% of black kids had parents who grew up in the top three levels and moved down at least one.

Let B = all black kids; BT = black kids who had parents who grew up in the top three levels; BD = black kids who(se parents) moved down at least one level

The only way that sentence can be interpreted in English is:

|BT ∩ BD| / |B| = 78%

If what they wanted to say was:

|BT ∩ BD| / |BT| = 78%

Then the way to say that in English would have been:

> Of the black kids whose parents grew up in the top three levels, 78% moved down at least one.