If you are disputing the accuracy of billings per se, that is done under the provisions of the Fair Credit Billing Act (FCBA), which provides a process for disputing any credit card billing issue, regardless of whether the creditor has reported to a CRA and it is thus also showing in your credit report.
The FCBA normally requires disputes to be made within 120 days of becoming aware of the billing issue. However, the credtior is apparently saying that they will not dismiss a billing related dispute becauser of timeliness of the dispute.
Disputes under the FCRA relate to the accuracy of their reporting to a CRA. If the billing related issue has also resulted in the reporting of information to a CRA, such as balances on the account, that you consider inaccurate, you can dispute under the FCRA.
The two dispute processes have different requirments and procedures, The FTC web page provides a summary of how to file a dispute under the FCBA, which normally provides for requireents upon the creditor that are not provided under the FCRA disptue process, such as providing supporting documentation in support of their finding.
