To "alarmists" both "climate change does matter" and "China isn't the only problem" are valid arguments because that's a logical AND: "it's a problem and we're causing it, so we should do something". When you inverse it you use DeMorgan's law and you have to disprove one, either "it's not a problem" or "we can't do anything to stop it" but they typically do it in a way where one purported disproof invalidates the other, for some reason. They argue both "it's not a problem" and "it's a problem but we can't do anything to stop it".