To me that email reads that they still require biometrics for your mother. It is possible that an officer just quickly scanned through your file and sent this letter without checking thoroughly if biometrics was done or not.My "Mycic" status doesn't say any documents require, I just got generic email for PCC, Country specific docs. , Passport copies, CV ect.(which is different letter). I have sent all documents to provided email but never received any conformation or update that ensure they received all document copies.
for Bio request is updated after Bio completed for mother.
Does your mother have a proof, e.g. A payment receipt, a sticker etc? I would submit that in response.
My parents received barcode stickers on their passport covers when they did biometrics for PGP application, and I submitted a photo of these in response to the standard doc request.