You haven't mentioned the complete sentence => "98% of Indian Population have not even heard the word "Github" in their entire life, yet everyone acts like they know this stuff".

The problem is "They try to act" like they know about software industry, meanwhile when you try to explain them that you are working on some open source project, they refuse to respect your opinion.

But 98% of US population, or 90% maybe - have never heard of github either.

Read the previous paragraph in the article. The point was that "everyone" assumed they understood how well he was doing, while being unable to evaluate that. (E.g. never heard of github.)

Why do they need to act like if the know github? What's important about github outside the dev community????

I guess he means for the people who commented on things he did, ie: his relatives. They did not know of his experience/the projects he had worked on - they did not know his "Github" work, and only recognized bigger brands & universities names.