The files in the Allen Cell Types Database are NWB v1 files. I don’t have direct experience with matnwb, but it looks like it may only support NWB v2 files. There are older versions of packages like pynwb that support the older v1 files.
If you want to inspect the data structure, all NWB files are HDF5 files, so you can use tools like HDFView to examine them.