Generally if someone is telling me how great they are, I assume they're not.
Yeah, I would be skeptical of any claim that conflates introversion, being humble, and having bad social skills, which are three completely different topics.
Also, if an employee is really really good, they will not make the negative curve of the graph. It is normally the fringe players who are (unfairly) impacted.
Introverts have physically different brain organizations and may grasp for words at times. They tend to dislike self promotion -- they perceive it as fake, and also dislike having attention drawn to them. They generally think a lot and say very little. They tend to focus on details quite a bit.
Introversion isn't a single dimensional thing, it's a cluster of dozens of tendencies brought about by this alternate brain organization.
Source: The Introvert Advantage (book).