Is it a buff?
I think the free trade routes is more flavorful and maybe more valuable.
Carthage was a classical-era coastal mercantile empire, and I think that is better replicated by the trade routes than by the food.
What I liked about the vanilla Carthage is that you could expand around the coast of a Mediterranean type region and never need to build roads to link up cities, as well as getting the economy boosts (and happiness boosts with Liberty) of trade route connection.
This feature is lost (or at least drastically weakened) in the current version, because the trade route building isn't available until a significant way into the tech tree, and you really need to get trade routes up and running before then from an economy perspective.
I would tend to change the ability so that it grants free trade-routes once more, I think that would be more fun.
I also wonder whether coastal cities are becoming too good for growth too early, and if swapping the buildings or their effects again such that the earlier building gives trade routes and later building gives food might be better.
The coastal-trade route building should allow you to skip building roads without suffering too much in terms of losing trade route access. At the moment I find that it comes a bit late.