No, this is the fallacy fallacy. You cannot conclude from someone using a fallacy that they don't also have a logical counterargument. You simply don't know. If you're playing the probabilities, yes, it's less likely that they have a good argument. But there are plenty of other reasons they could use a fallacious one instead. (They're upset, they don't fully understand the distinction between good and poor arguments, they aren't viewing it as an argument but rather as a flamewar, etc. etc..)