If you are using the indoor module, then you would have to use the Azure Maps Creator platform which is only available with a S1 license.
If you have your own tile layer or some other form of indoor data that you can overlay without the indoor module (which only works with the Creator services), then you could connect to the data within intranet without connecting to any Azure Maps services. You would only need an Azure Maps account if you wanted to show base maps. If not using base maps, I recommend setting the map style to "blank" and set the "enableAccessibility" option of the map to false when loading it so that the screen reader doesn't try to call Azure Maps services. If you do those two things, then no Azure Maps service would be called and thus no Azure Maps key would be needed to use the map. You may see an error message in the console about the lack of credentials, but the map should work fine.