Hi @lindsaymiles,
I have to start with a disclaimer: the .NET MAUI framework is constantly evolving so things may have changed since this article was written.
Have you tried downloading, configuring and running the sample project?
Related to your issues, the protocol/scheme declaration for Windows and the WebAuthenticatorCallbackActivity subclassing for Android should have been done as described in the Apply Platform-Specific Changes section of the article. Did you do apply these changes?