Maybe I'm wrong but doesn't this put it in a totally different category than other fallacies? Circular arguments, for example. All circular arguments are wrong. If you identify a circular argument you don't even have to fully understand what is being said, you can immediately conclude "this is a bad argument".
But appeal to authority isn't like that. "I'm not going to drive through the mountains today because the roads are iced over, and I know that because the highway department said so." That's an appeal to authority, and it isn't proof, but it's a good argument and there's no need to drive out yourself and confirm that the roads are dangerous. Identifying an appeal to authority is not enough to discard an argument, you need to evaluate the authority. When you see a circular argument you don't have to measure the diameter of the circle or something.
So I don't think appealing the Grammarly was a fallacy, but in this case I think they're wrong.