I have used this code with imported datasets in Power BI as well as datasets that use DirectQuery against a Azure SQL database. When you have an imported dataset, you do not need to patch the data source credentials to make thing work at first because the data has already been imported into the Power BI cloud. Therefore, you only need to patch credentials for an imported datasets if you want to be able to refresh the dataset on demand or to schedule data refresh inside the Power BI service. DirectQuery datasets do not have imported data so you have to patch credentials for them to work at all.
This code was a bit confusing to me at first because you have to create a new GatewayDatasource object even when you are accessing an Azure SQL database without going through a Power BI Gateway.
My understanding is that this programming technique is also supposed to be used for datasets based on a live connections to a tabular database in Azure Ana;ysis Service. However, I have never tested it myself so I don't want to say it works until I have seen it work with my own eyes. I would love to hear from anyone in the Power BI community that has patched credentials to a Azure Analysis Services database.