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If you didn't sign up as a co-borrower, then it shouldn't be on your report. Dispute it immediately.
I would not file a dispute.
They need only verify what they have already told you is their interpretation.
You have an issue of factual interpretation of what is or is not included under the FCRA.
That is fodder for ruling by the CFPB,
I would recommend handling by way of filing a complaint with the CFPB, who has clear authority for administrative enforcement of the FCRA, and get their interpretation.