Polders are situational, especially since marsh is much rarer than hills or mountains. Even if the Dutch get a marsh start, they're still only likely to get 2 or 3 polders out of the deal. Plus they don't synergize well with anything else about the Dutch, unlike terrace farms which build on a strength that the Incas already have.
Terrace farms aren't just extra food, they're extra science and specialists earlier and quicker. Plus you can use this to work high production tiles(mines on hills not adjacent to mountains, for instance) faster and sooner. We aren't insincere. The Inca can put out massive amounts of food and production very quickly.
Okay, here we go again...
The Marsh is much rarer than hills or mountains, yes, but remember Polders can also be built on Floodplains. Once built, they immediately add +3 food (technically +4 for Marshes) which means that a single tile with a Polder grants 5 food. As you yourself stated, more food is more science and more specialists, and the Polder makes up for the rarity where it can be built by being so ridiculously powerful once constructed (that +1 Production and +2 Gold post-Economics is just a little extra goodness.) You need 3 mountains adjacent to a riverside hill tile for the Terrace Farm to grant the same amount of food prior to Fertilizer, and that's a condition far rarer than a Marsh or Floodplain. And sure, you can spam Terrace Farms on hills not adjacent to mountains - for a 1 food, 2 hammer tile. An improved Sheep tile is better than that (even after Fertilizer), and it just isn't worth sacrificing the extra hammers. Also, in the games I've played as the Inca, if I'm lucky I'll get one or maybe two hills near my start, and those are rarely near mountains, so, like with the Polder, you pretty much need to wait until your second city to start building these tile improvements to any advantage.
Now, I still don't understand this argument that the Terrace Farm synergizes well with the Incan UA. Yes, it certainly synergizes better than the Dutch UA and the Polder, but that still isn't saying much - the Incan UA lets you move onto a hill tile and start a Terrace Farm in the same turn, but that's it. Like I mentioned a few pages back, the Terrace Farm and Incan UA synergize like the Moai and the Polynesian UA - your start bias will, in theory, allow you to build more of these improvements, but otherwise...?
So yes, the Inca can put out vast amounts of food and Production very quickly - given the very specific proper conditions. In regards their UI, they're simply no better than the Dutch.