Kappell did not “say” the slur at all. Rather, he committed what linguists refer to as a speech error. That’s the jargon we use to say: He made a mistake. And no, the mistake was not in having the gall to call Martin Luther King Jr. a coon on the air, but in producing a vowel he did not intend that accidentally sounded like the slur.
A little more jargon: Kappell experienced a typical perseveration, a sound from earlier in an utterance that holds on into a later part, like a bit of food may get stuck going down for a bit. It happens to all of us—you say black blox instead of black box, or Tom gave the goy a ball instead of Tom gave the boy a ball. With Kappell, the oo sound in Luther held on for a bit and bumped out the ee sound in King.
Some may feel that judgment must be different when it comes to race issues. However, there is no reason that an ordinary linguistic phenomenon like perseveration would somehow fail to occur when someone is referring to black people or other minority groups. Human beings can never have perfect control over their running speech, as we know from the small speech errors that all commentators make all the time.