Is it his knowledge of C# or Java Syntax? Is it his knowledge of every corner of every obscure framework? I don't think so. It is important to know what is possible in a language or framework but you can always lookup the fine details when you need to.
This reminds me of a story of when one of Einstein's colleagues asked him for his telephone number one day. Einstein reached for a telephone directory and looked it up. "You don't remember your own number?" the man asked, startled. "No," Einstein answered. "Why should I memorize something I can so easily get from a book?"
It is the developer’s knowledge of good object oriented design principles and domain modeling that makes a great developer stand out. If you are not strong in these areas it will be very difficult for you to Google them as you go.