That is how K apertures work: diffraction. But in optics that is how lenses work to either focus or disperse light into a wider angle. It works like a combination of an acoustic diffraction lens to provide a very wide polar dispersion, but has some coloration in the form of dips (for large K aperture). For a small K tube, it appears that the effect is mostly the dispersion without much coloration. The gradual slope and expansion acts as a smooth impedance matching device and gives high efficiency for coupling like a horn without being big like a horn. It looks like it would mess up the sound - but test the K tube and you will be pleasantly surprised that it is one of the smoothest sounding CD waveguides out there. Same with K aperture on a full range speaker - there is a reason the mini Karlsonator is so popular (dozens of happy builds) - it sounds great. So skepticism is good but not always correct in prediction of outcome.