hi,
that particular BIM asset from Cesium Sandcastle
is in 3D Tiles which uses B3DM format for its content children. I assume you have converted one B3DM file to .glb. using 3d-tiles-tools as I suggested in Searching for local .glb file 3D Tiles BIM - #2 by xmitter
If you have used the convertB3dmToGlb then it will store the attribute data from feature and batch table into resulting glTF (.glb) using EXT_mesh_features and EXT_structural_metadata glTF extensions.
If you have used the b3dmToGlb command then it just “Extract the binary glTF asset from the input b3dm.” without attribute data.
Also be aware that the asset contains multiple B3DM content files which can for example provide better LOD.
I don’t really understand why are you trying to do this with that particular sample asset, it would be good if you could give some more information what you are trying to achieve.