It's a shame that the meows in the "meow-y" setting are just regular meows, not purry meows (as in, meowing and purring simultaneously).

here's some more purrs: https://mynoise.net/NoiseMachines/catPurrNoiseGenerator.php

OP's post was amazing but can someone explain how this was probably made?

I think the sound is generated locally. Check out the page source, which contains the JS I think is responsible for generation (I haven't checked in detail).

captive feline

Meow-generator when the last slider is set to Meow-y ;-)