An additional wrinkle with Thornton is that his velocity keeps climbing. So more or less IMO Thornton has progressed from 'bad and super lucky' early in the year to 'okay and kinda lucky' mid year to 'maybe really good actually' over the last few weeks?
Thornton through 6/16 (his first game back after Pat leave):
94.8 MPH fastball, 1.86 ERA, 5.14 xFIP, 26.6% chase rate, 90.5% z contact rate, 89 pitching+
Thornton 6/20 - 8/7:
95.1 MPH fastball, 3.71 ERA, 5.04 xFIP, 35% chase rate, 87.2% z contact, 99 pitching+
Since 8/11:
96.2 MPH fastball, 0.00 ERA, 1.59 xFIP, 31.8% chase, 80.8% z-contact, 115 pitching+
So basically early on he was terrible and it was pure smoke and mirrors. Mid year he improved as his stuff started coming back. I think that xFIP probably understates the improvement, 35% chase is stellar and he probably got some legitimately earned soft contact that would have regressed to more Ks.
Lately though? He's just been awesome. I don't want to get over my skis over 5.1 IP but holy cow he looks like a different guy.