Hi,
The asset endpoint route can return two different values. One is for Cesium ion hosted assets and the other is for assets hosted by external providers (such as Google). The options are slightly different for each type.
I can see how how this could be easily overlooked since the responses are similar and there is not a field that explicitly identifies if it is an internal or external asset.
Here is a screenshot from the relevant section of the API documentation for the external asset response:
Hopefully this information will help CesiumIonLoader fix the issue on their side so they can support both internal and external assets.