NextJS Auth: Where should the session cookie be set

From NextJS [docs on authentication](https://nextjs.org/docs/app/guides/authentication#:~:text=Cookies%20should%20be%20set%20on%20the%20server%20to%20prevent%20client%2Dside%20tampering.) it is advised to set the session cookie on the server side. However in this the guide provided here, the cookies are set on the client side. What are the consequences of setting the cookie on client? Is it a security issue?
https://github.com/firebase/friendlyeats-web/blob/6bf6c0e7f3c480c6ec4675814351e5a254ca092c/nextjs-end/src/components/Header.jsx#L17