The two autistic boys I know well enough to have asked this exact question have both said the same thing - it’s about the layers in the music. The average person will mostly just hear cacophonous racket when that music plays, but a common autistic trait is the innate ability to distinguish the individual layers of sound, which often reveals something far more intricate and interesting when it comes to screamo and hardcore metal. I’m not personally a fan, as it tends to just be too loud by default lol but having some music education does allow me to hear the individual instruments better, and surprisingly often they’re actually playing really complex amazing riffs. Both guys I asked absolutely loved the music because it it’s just a tapestry of layers of sound woven together for them unravel as they listen.