The potential artistic value here doesn't appear to be so much in the pure number of notes, which is not particularly impressive in itself, given that MIDI file

The potential artistic value here doesn't appear to be so much in the pure number of notes, which is not particularly impressive in itself, given that MIDI files are easily auto-generated. But the examples given remind me most of all of the old early days of overdriving vacuum-tube based amplifiers till you got the crunching of the waveforms and created an entirely new sound. Here, the folks creating these files are creating new kinds of sounds by overdriving their MIDI synths, essentially. By providing more notes than can actually be generated at once, or at rates faster than the chosen synth patch can coherently render, you can achieve new and unplanned sounds and effects. Essentially the same idea as getting the explosion-type of sounds out of old 8-bit PCM synths in game consoles by overloading the input beyond spec.