It wasn't bugged, it was just mathematically impossible due to HP vs damage done and timers.
C'thun was never actually proven impossible and knowning what we do now it's very likely that it actually wasn't.
The amount of trash being talked about this kill and about how Classic is "so much easier" than vanilla is kinda sad.Less than 0.1% of the people talking (myself included) could pull this off.BoK deserves as much credit as can be given to them for doing this.