They might have used a threat securing goop, like locktite.
Try soaking the screw in acetone and hope it gets down into the thread area.
Then use an impact screw driver, the kind with internal weights, that you twist while banging with a hammer.
Another method requires having a drill press, and hand turning the press head while applying pressure with the handle.
Walt Kelly, alias Pogo, sez:
“Don't take life so serious, son, it ain't nohow permanent.”