Sarcasm is a tool for someone that can't actually come up with a counter-argument. It's an appeal to absurdity combined with a strawman. Exaggerate the other side's point until it sounds ridiculous and pretend that's an argument.
The sarcasm seemed to be based on the fact that you can agree that truth is good and falsehood is bad and not see that as a reason to dismiss the Guardian piece.
"Opinion piece is misleading because it isn't a factual non-biased piece of journalism" seems like one of these times.