Someone else has already addressed that this aired well before 01 April. If I recall correctly (I listen while driving to work) this was one of the Story Corps recordings. A google search gives this website for the Story Corps project:
http://www.storycorps.net/ .
Essentially, mobile recording booths rotate around the country and people can record whatever they feel like recording. Some of these get played on NPR. I think the ultimate destination of these recordings is the Smithsonian or something, but I do not know for sure. I presume that the Story Corps website will give more details.
Since these are literally stories, as opposed to produced news, there is no journalistic requirement for veracity checking. If a story is not true, it should not be construed that NPR has any responsibility for the untrueness of the story.
Further, if the story is untrue, the teller would have not only made up the story but also gone to the trouble of seeking out and taking time to go to one of these Story Corps setups. Yes, it is possible that this is a prank, but going to all this trouble seems a bit much to me for a prank, so I tend to believe the story teller.
Regarding the language that the story teller gives for his assailant - we have no idea what the background is of the teller of the story. A journalistic background is not to be presumed - the teller could be a pipefitter for all we know. How many times have any of us recounted what someone else said as an anecdote and given a verbatim account of what the other guy said? This is exactly why hearsay is generally not admitted as evidence in court - we all tend to translate others' words into our own personal language. What we think the other person "means" is more important than what they actually say.
There is no gun control message in this story. This sort of reaction may well be useful for a CCW holder in some instances, especially in light of how if a knife is produced within 25' or so the CCW holder is pretty much SOL anyhow. Each potential situation is different - the story teller sensed that he could handle the situation the way he did and it worked out for him. Surely we pro-gun / pro-CCW folks are not trying to dictate that violence MUST be responded to with arms? Surely we won't say that there is not a place for behavior as demonstrated by Gandhi, the Dalai Lama, Jesus Christ and countless other figures espousing non-violence as a choice rather than an imposed rule?
I don't know that I would have reacted the way the story teller did - rather, I probably would have fought. But, the right to bear arms is a right, not a requirement. Responding to violence either in kind or with active non-violence like the story teller did are both valid choices. Indeed, both choices must be available to the individual if we are to maintain a free society.