In the long run immigration and diversity are likely to have important cultural, economic, fiscal, and developmental benefits.
Did you notice anything strange about that quote? It's especially obvious when you read the whole passage...
What he's basically saying is that evidence shows how diversity hurts a community, but in spite of that he thinks that at some point in the distant future, it will probably somehow turn out to be beneficial.
So he has enough intellectual honesty to admit that the facts contradict his beliefs, but then he lets his biases get the better of him and comes to a conclusion that supports the dogma of diversity.
You're partially right, there's a contradiction there, but it's only in his head.