I know that. I play the piano. I think I even qualify as a "fairly experienced singer and pianist" [1].
As others have pointed out, there is structure here beyond the fact that the tones are constrained in a way that results in harmonious intervals. This isn't random wind-chimes. Specifically:
* The mind tends to hear lines in the music, associating notes with particular preceding notes to hear melodies embedded in the polyphony. These lines move together in interesting and (to me) pleasant sounding ways, some moving together at fixed intervals and some moving in contrary motion. The avoidance of consecutive fifths and octaves in counterpoint is to prevent lines seeming to blend together into block chords, but here that happens a lot so the lines don't "live" for long. They briefly pop out of the harmony and merge back. There are many ways to hear that depending on what you focus on, so there's an ambiguity/interplay between hearing separate lines and hearing chords that tickles my brain pleasantly.
* The fractal is built step by step (in two different ways if you listen to the whole thing), which creates a rising/falling/growing pattern that builds a pleasing tension and release. It also provides a rhythmic framework and emphasises the self-similarity, as the previous pattern is repeated while further similar copies are added over the top.
Sierpinski's gasket isn't a picture. It's an abstract mathematical object. The image and the sound are both imperfect physical realisations of that platonic ideal and are directly related to each other by a very simple mapping. You could certainly argue that in the sound you are "hearing Sierpinski's gasket" to the same extent that you are "seeing Sierpinski's gasket" when you view the image. You'd be on far stronger ground than people who take physical phenomena such as black holes that in no sense "sound like" anything, arbitrarily map some aspect of them to a waveform and claim the result is the "sound of" the original phenomenon.
But the post doesn't make any such claim. It just points out that you can use the fractal to make something that both looks and sounds cool. I agree.
[1] https://news.ycombinator.com/item?id=34132576