The distortion sounds are being distorted on individual strings, as opposed to after summing. If you look at the credits, and follow the link to the sound sources, you can find the files used. If you listen, you'll be able to match them up. While I agree that the distortion should happen after summing, I suspect the playing and summing of individual already-distorted string sounds was done because that's what could be implemented simply and easily. Applying distortion after summing may well require more extensive coding, or stuff that's computationally intensive enough to slow down the browser (I don't know this for certain, I'm guessing).

I'm not faulting the author for this, I probably would have done the same.