It is a convention that when you say "written in bash" it means "using bash language and standard *nix binaries". Bash alone can't do anything, nearly every keyword in it is actually a binary, including "echo", "true" and "false" or "[" used in if statements, which you can usually find in /bin or /usr/bin. You couln't write simplest bash scripts without using any binaries.

Netcat (nc) is built in to nearly every system that has bash. Wget and Curl on the other hand, are not.