Did you really expect them to fund you with just an idea and some domain knowledge? Not to sound rude, but that's pretty naive. How many people are out there who have a "million dollar idea" who just need someone to build it? The answer is a ton and a boatload of them apply to YC every year. Good ideas are abundant. Proof of concepts and prototypes are not.

Getting into YC does not mean that Modular Science is going to be a success. They still need a good tech team, people with domain knowledge, and a market to sell to. Your farming knowledge is definitely important to a project like this, but there is a much greater supply of it. It wouldn't be that hard to hire a couple farmers to be subject matter experts. It is and continues to be insanely hard to put together a team that can build autonomous robots.

Don't give up on your dream, but you need to realize that a farming robot is technology. That's what you're selling. You need people who can build the technology. Find them and build the company. Make a prototype. Use your domain knowledge to make it better than the competition's product. Don't just be a guy with an idea, be a guy who builds things.