So you're saying it only counts a resource of yours if the destination city doesn't have it?
The thing is it also counts resources which the destination city has that your city does not have.
To clarify, it counts for both the resources of the home city and the city of destination. If Lisbon has horses, gems and gold while Ife has iron, horses and silk, the resources that would be counted in resource diversity are gems and gold from Lisbon and iron and silk from Ife. Even if you placed a Feitoria in Ife, the resource diversity remains the same--you won't be getting the silk from Lisbon and thus cannot be removed from resource diversity when trading between the two cities.
I also think this civ is one of the worst in the game. As others have pointed out, the extra gold from the UA seem negligable. Both their UA and UU has strong Gold flavor, which means pretty much the only victory condition you have an advantage for is Diplomatic Victory, and that effectively renders your UI pointless. If Naus were at least available eailier than Astronomy, they would have an advantage on continents map that would mirror their history of being a strong civ for exploration.
Gold is also useful for research agreements, purchasing buildings and units, paying maintenance costs for larger cities and armies, buying resources from other civs, bribery, and with Freedom, purchasing of spaceship parts, in addition to influencing City-states for their bonuses. Hardly just for Diplomatic victory at all, although it most certainly is the victory it is most associated with.
The UA is, in-fact, not negligible at all, especially if you've been getting techs and wonders that grant trade routes. So you managed to get four trade routes which grant you four cargo ships, netting you 10 more gold for 30 turns (300 gold). That already pays you for your research agreement, or perhaps helps you purchase a crucial building faster while you're building the Sistine Chapel. I think a better argument for the UA is that internal trade routes are also very important, not so much that it's negligible. Fortunately, their UU allows an alternative source of gold, allowing you to focus on internal trade routes during this time.
Feitorias also allow you to do something that others couldn't: demand tribute from CSes or even go to war with them while still gaining the benefits of their luxuries. Tribute means even more gold, but if it means maintaining your larger armies, you can definitely take over the world without needing to worry too much about happiness. It's true they're less useful in Diplomatic victories, but if you're not focusing on allying City-states, you won't be needing to pay for them for their luxuries just to maintain your happiness.