It really was. But it's possible the volcanic lake only projected a captured cell of hot air up to the crater rim. Had they flown over the prison and then directly down they might not have crashed. Though they would still be trapped.

If only Zuko had his fire fists or that of his sister Azula at that point it would have been easier.

Man Uncle Iroh is OP with the fire breath technique he teaches Zuko for the "chillers"