To check if polygon offset is supported you can add console.log(viewer.scene.shadowMap._polygonOffsetSupported); or step into the code here:
We also apply normal offset bias, which helps, but there are still shadow artifacts. It looks like your model does have normals and can use this technique. You can try tweaking some of the values here and see if it helps:
What device are you running on? Some mobile devices have less depth buffer precision. We’ve seen issues like https://github.com/CesiumGS/cesium/issues/7176 and https://github.com/CesiumGS/cesium/issues/4752