He's a library designer and is working with a team maintaining and updating one of the most popular Javascript libraries in the world. He has a very deep background in that language which is one of the most important skills for his role.
I don't blame him for not knowing some of those things you listed more deeply. There's only so many hours a day to dig deep into a wide range of material. You still have to spend time going into other material that might be more relevant to the day to day including testing, managing people, communication with team members, planning, etc. Yeah sure I'd love to be more knowledgeable about CORS or networking, but I also want to be much stronger in Javascript, the JS library I'm working on, design patterns, deployment, etc.
Lastly, I'd rather be in Abramov's shoes where my work profoundly influenced web development than be all over the place with my 'expertise' and not actually be all that impactful with my time.