The guide you linked is for Gitea acting as the authentication provider (like how you can login to this site from GitHub, GitHub is the authentication provider). I think you may be misunderstanding its purpose.
The redirect URI in this case would be the application you are providing authentication for. So, if I made an app and wanted to login users with Gitea, I would do this:
User clicks login button. My app redirects them to my Gitea instance to login:
User logs in on my Gitea instance. Gitea asks them to authorize my application. User authorizes my application. Gitea redirects user to my application with the authentication data (in this case a code):