But it may still be a valid question, you just would not expect the same level of detail. I do believe that developers should at least understand the basic concepts of how the web works. Maybe not the structure of a TCP packet, but certainly how DNS and IP addressing work, and the flow of data between a server and a client as web/mobile apps are loaded.
Why would you give a "FizzBuzz for sysadmins" question to full-stack developers though?