It’s called nothing in that quote that says he never heard of the KKK.
That was an answer to a question about David Duke.
The thing is - he has heard about David Duke and has disavowed him twice. The first was in 2002, the second was as recently as 2 days ago.
He could have handled this much better instead of pandering to Duke/stormfronters. He could have turned this into a discussion of La Raza instead.
Shameful behavior by the front runner.