It is clear that he wrote that the new district will not nominate the Democrat preferred by 85% of Blacks ["the candidate of their choice"] if Russ Carnahan runs.
He didn't write "they won't nominate Clay if Carnahan runs," he wrote "let's assume this, and then fudge some numbers there, and wave our hands a little, and then assume some more, and maybe Carnahan has a chance to win." Note that this guy appears to be the only Democrat who believes this is possible, and he's not even saying he'd win.
This is somewhat less defensible than my quoting one of Ron Paul's newsletters as saying "White Republican Let's Cat out of Bag about Republicans hating African-American's."