This is an excellent feature that finally gives us a way to generate Treasure Fleets without settling/warring. However, it is arbitrarily gated behind a Diplo City-State bonus, as a result only one player in the match can access this feature.
I suggest it should be available to all players somewhere in the Civic tree (Diplomatic Service? Imperialism?) and the City-State bonus could maybe be renamed "Gunboat Diplomacy" and enhance Demand Tribute to be faster or give more points.
While we are on the topic, a friendly version ("Treasure Purchase"?), usable with Allies which gives the target Gold before spawning the Treasure Fleet would similarly be nice to have.