This isn't useful for several reasons:
1) Nothing strange happens. The shine is disrupted. We've seen this before. If you don't pull out the shine fast enough and you're hit, a strange blue hexagon surrounds you and you get knocked back. (Happens to me all the time with Lucario's Aura Sphere). In this case, Falco's Reflector isn't disrupted by a projectile, but a fist. DK's fist touches the reflector and causes DK to get hit. This results in both characters getting knocked back.
Because DK only gets hit by the reflector, his knockback is weak. He moves slowly. Falco, who got punched, flings out of control and slams into DK hard enough to send DK into KO-zone. Falco then flies off the screen. Watch the video again, and notice how DK would NOT have been stocked had Falco not knocked into him. His knockback was too weak.
2) In other words: Falco doesn't even stock DK. Well, not with the reflector. Which means ultimately, this situation forces Falco to commit suicide, while DK lives.
3) We don't start matches at 300%, most respectable tournements don't use 'sudden death' mode...and if you do manage to get your opponent past 300%, you have far more reliable options. There's no reason why this should ever be done.
4) This isn't an AT. It's just the results of what happen when Reflector and DK's tilt (jab?) hit each other.