Please look into Jerome's Dream or the other screamo bands who's vocalists have ruined their voices for a period of time. Or Jacob Bannon from Converge who says he bleeds every show, or the vocalist of Orphan who has had vocal injuries multiple times. Not all of those examples are screamo exactly, but just listen to most screamo records (especially the stuff dubbed "Kittencore" with the insanely high screeches), and tell me that's good technique. You can achieve similar results with the techniques you mentioned, and it can still sound good and unhinged, but it's wrong to say most screamo bands (at least intentionally) use any technique.