They use it live.To the extent that you're interested, try Youtubing up the live performances of that song or others by him. Most of my exposure was on the tv show itself, where I assume they haven't had time to electronically doctor the performances.
OK, I just listened to him doing an a cappela bit of Bohemian Rhapsody, followed by that thing by Four Non Blondes.What you say is right: there are times that he and others don't quite get the note, but I think it's possible to overlook a minor slip or two, if the overall performance is good. It seemed to me that his performances were very good, with or without the doctoring.
His intonation is poor. Lots of passion, lots of pyrotechnics, all the usual prettyboy stuff.
But he's not a good singer, even if he performs well.
ETA: just listened to a studio recording of "Mad World". I couldn't say for sure that he's got pitch correction on there, but the pitching is almost too good to be true, and the vocal has a rather over-processed sound to my ears. Not bad, though. But...well, nothing outstanding.