Well, I guess the two most likely scenarios are:
1)Moz's team were spinning old Cliff a bit of a tale, making Moz's illness sound more serious than it actually is, to soften the blow of the cancelled concert. I mean, if Cliff asked how Moz was a couple of days ago, Donnie can't exactly say "Oh, he's fine now. Fully recovered and waterskiing in Miami" can he? He'd look like a right idiot. He'd have to at least pretend he's still at death's door.
2)He really IS still in hospital. Two weeks in hospital trying to recover from a virus is pretty severe (if you believe KY really was Patient Zero, she seemed to recover in 48 hours), but the guy has a LOT of underlying health issues.