In many ways, we still behave like animals and react to social cues as such. If we can't understand a person or decode where they're coming from by reading their body language, we will probably dislike them. That is a fact, and no matter how well you do your job, if you don't have charisma, you will likely spook your coworkers and end up losing it. I don't make the rules.
Maybe if you're sitting at a desk all day in your house, and your job doesn't require you to show your face or talk directly to other people, you can excel; but if you have to share with other human beings, you have to get at least a little bit good at socializing.
If you are an introvert reading this and you realize you've been having this issue throughout your work, maybe it's time to change careers. I don't mean this in a bad way; what I'm saying is we all have a place where we can shine, and I think, for introverts, it should be places that don't require them to be more social.
It's fine to try to get better at things that we're not naturally good at, but we should know our limitations. Maybe we could be much better, or at least happier, in a role that only requires our work and doesn't come with any social expectations. Luckily, we live in a day and age where there are many. I'm sure back in the day this would've been impossible. Even if we were lonely writers, we still had to talk to editors.
So if you feel identified with this employee's story, maybe it's time to change your career path! Some commentators on this post are giving the quiet employee the same advice but for the wrong reasons, they say being social is not a real requirement on any job, and I couldn't disagree more, being human is not a disadvantage!