When I tried to run your given repo in my local, I got the below warning.

If you want to add the Web.config file to Angular App before deploying to Azure App service, then follow the below steps.
Open the Angular Application root directory.
Add the web.config file in the src root directory.
Open the Package manager console and run ng build.
npm install
npm install --save-dev @angular-devkit/build-angular
ng build
- A new folder with name
dist will be created.
Now re-deploy the app again.
Is there a way to modify the web.config through the Azure portal.
If you have deployed your application to Azure Windows App Service, then we can create the web.config file within the deployed Application Angular root directory from KUDU Console.
Open the KUDU console by using the below URL,
https://YourAppServiceName.scm.azurewebsites.net/


Make sure you are creating webconfig file in the Angular Application root directory.
If your application is deployed in Azure Linux App Service, then you need to create .htaccess file as mentioned in the Link which you have shared.
To deploy the Angular app and .NET Core API to Azure App servicewithout any issues, follow the SO Thread 1 and 2 for more details.