Yes, "main character" is Twitter jargon not normally used in business.
However, it is true that the manager often represents the team in meetings. To the other people in those meetings, the manager may be the only person they know on the team.
An extreme version of this: outside a company, the CEO may be the only person the average person has heard of. Who do you know who works for Tesla, besides Musk?
Or consider the inventions attributed to Edison. Could you name anyone else who worked in his research labs?
Matt Tait talks about Twitter and its main characters here[0]. I just block all these 'characters' so I never have to see them in my feed. And it's not just a handful, I block ~300 accounts which in my eyes are all bad faith actors / troublemakers who are subtly trolling Twitter because they have a large following.
It's inspired from literature. It means someone who views themselves as the main character of the world's story. In some literature, the world tends to revolve around the main character. (This is not true of much of literature that deals with, for example, character studies, but think of books like Ender's Game or Lord of the Rings.)
Although we are each usually the 'main character' of our own story, we typically know that we are not "the main character" of the world broadly. Some people act in ways that suggest they may not understand the difference. People refer to them as having "main character syndrome."
IMO the people that throw around phrases like narcissist, main character syndrome, lack of empathy, etc. usually know nothing about the person they are attempting to diagnose and simply have differing perspectives that they can't see eye to eye on.
On Twitter, I think it means "someone who thinks they're the main character of the story"; that is, someone self-important or narcissistic. But I'm not a Twitter person (a Twit?), so I'm going by what I hear from others.
On Twitter I'd say the "main character" designation is less about how a person acts, and more about how everyone else is talking about them. They are the main character of the moment because suddenly half of your timeline is other Twitter users either directly or indirectly referring to them. Usually because they are wrong in a way that drives a ton of engagement.
However, it is true that the manager often represents the team in meetings. To the other people in those meetings, the manager may be the only person they know on the team.
An extreme version of this: outside a company, the CEO may be the only person the average person has heard of. Who do you know who works for Tesla, besides Musk?
Or consider the inventions attributed to Edison. Could you name anyone else who worked in his research labs?