Correct me if I'm wrong but I think PLM/PDM are storing the CAD model as "CAD databases" (basically the proprietary file-format) in a Data-Vault. So while not all CAD data is being stored in those files (meta-data for example) those old file-fomats are still in use I think.
TBH I don't have a great understanding of how the PLM systems store CAD data.
What you describe sounds about right, though. I think in some cases it really is just the original CAD file, as a binary blob, in a database, with metadata stored with it. In some cases it's more complicated, though.
Catia's V6 "3D Experience" system, for example, saves to their database by default, but allows exporting to Catia V5's CATPart/CATProduct format, or STEP, for data exchange. The catch is that some of the newer features in V6 don't exist in V5 and aren't fully supported by CATPart/CATProduct, so they have a way to export those bits "as result" instead of saving the full dataset - basically decoupling it from the parameters and inputs that created it and giving a BREP blob. In most cases the exported parts can be editted in V5 and sent back to V6, but the "as result" parts can't be.
It's different for every CAD vendor, though. A nebuluous mess of vendor lock-in and proprietary data formats.
You can also do PLM/PDM by exporting the CAD model in a neutral format such as STEP and storing that in a database.