A fiduciary holds property for someone else and owes them duties of loyalty and care. National banks get that authority from section 92a of Title 12, and a trust bank's application to the OCC includes a request to exercise it.
Block's application cites section 92a and section 24(Seventh) as the basis for custody, riskless-principal execution and stablecoin settlement. The request itself is filed as a confidential exhibit, which is why the public record shows the powers sought and not the mechanics of how keys would be held.
The word matters because a fiduciary duty is stronger than a terms-of-service clause. Whether it displaces a clause that puts theft risk on the customer is a question only the approved operating agreement can answer.