Currently Dash Prime/Apex. But short stock rebate has been listed in the past with all my old brokers, Goldman, Merrill, etc. I think your original explanation is probably closer to the case. Short stock generates cash, a credit balance which you are paid on. The short stock rebate category may include the credit balance interest received less costs of shorting?
This is IBKR's page re: shorting fees and rebates Interest Paid to You on Short Sale Proceeds Cash Balances Examples | Interactive Brokers LLC