That's true, you could not use the ToolNode object and define your own logic to handle tools. You do need to use at least the @tool decorator in order to attach those functions to a model using LangGraph's bind_tools function.
Defining some of the functions as tools was where some of the "self" parameter issues mentioned in the TDS write up came up - since those parameters aren't added by the LLM, but LangGraph/Pydantic errors if there missing.
Technically you could forgo using the ToolNode object, not define your functions as tools, not use the bind_tools method, and potentially not use Langchain's model abstractions - but at that point, you're not using much of the framework at all.