I was about to comment the same thing. Usually I don't call the function directly, but via the tty command in my shell scripts:

  if tty -s; then
    echo "Standard input is a TTY (interactive mode)."
  else
    echo "Standard input is not a TTY (e.g., piped or redirected)."
  fi

Now I wonder how _isatty_ itself detects whether a file descriptor is associated with a terminal!