As mentioned in the article, it's because it's based off of Paul Ekman's facial recognition work (FACS), which infers a person's emotional state based on their facial expression. The only reliable markers Ekman found were for those 6 emotions (and contempt and perhaps 1-2 others). I haven't heard of any for boredom

The misattribution of anger you're describing is pretty common, I think. I'm guessing it's because you furrow your brow in deep concentration (and possibly tense your lower eyelids), which is also common in anger faces. The real giveaway here should be the missing upper eyelid raise (and possible pupil dilation) that you won't be doing but should be there if you're angry. People and AIs that are trained with good visibility of your face shouldn't make these errors very often.

Our biology is hardwired to communicate the emotions we're feeling via our face. We want other people to know how we're feeling because it's important. It could be misused, though (Ekman's FACS work with airport security to detect lies was a failure).