I'll suggest to you that the boundaries between introversion vs mental illness are probably quite blurred in some cases.

And although it may seem ridiculous to you, there are people for whom board games and Russian literature are pretty stressful and anxiety inducing in the same way that public speaking is stressful for many people.

If you had also included math in your list of frequently disliked subjects, then it might be less ridiculous an idea

It's important to remember that stress responses vary widely

They said talking to another human being fills them with terror.

Blurry boundaries or not, either you're dedicidng to minimize what it means to feel terror, or I agree with who you're replying to. Bit weird to point the finger at someone and tell them they have mental illness, but it does look like it. Again, if you accept the description, I cannot see how terror would be an appropriate response to a conversation and would prevent someone from leading a normal life.

It would be nearly impossible to have a job for example.

Please do not impose the "mental illness" label on people over the internet. One of the foundational requirements for any diagnosis of a mental illness is a negative effect on the person's capacity to function, and you just can't evaluate that over the internet.

Its quite harmful to stigmatize peoples stories. Let people be different.

If someone is feeling deeply anxious over a social interaction, that’s a pretty clear negative effect on their capacity to function.

If someone never feels deeply anxious over a social interaction, that's a pretty clear negative effect on their capacity to function.

And how do you know that the Aditya from the story didn't have a mental illness?

I don’t know that. Why would it matter?