This makes no real sense to me.It does occur to me that the bribing officials in Traveller is a bit of a red herring. The fix is in there as well, because that rule is built in because there's a omnipresent corrupt bureaucracy that player characters will often have need to engage as part of play. So the bribing rule is only in that context against that much larger enforced setting and operates as a mechanical relief valve on that constraint. Fiat rules that deal with fiat setting restrictions aren't necessarily the best examples of things that increase agency. They preserve agency against setting constraints, but don't increase it.
It's like saying that combat rules in D&D don't give players any agency - they only preserve agency against the constraint that, by default, a sword thrust in the fiction can be deadly.
All action delcarations presuppose (i) a fiction, and (ii) some way the player wants the fiction to be. (An exception to (ii) is an action delcaration whose purpose is to get the GM to narrate more fiction. But these don't manifest player agency at all, except in the very weak sense of triggering the GM to say more stuff.)
In Traveller there is an implicit setting, which includes officials, and law levels, and the possibility of bribing people. That's (i). And the players can engage with that fiction, and impose their will on it (this offiical will let us through, because I've bribed her), by declaring actions and then getting good rolls. That's the player succeeding in relatoin to (ii). That's agency, in the context of the RPG.