I don't believe that YC made that assumption when they accepted the form builder. There are many things that's in-browser video editing app can evolve to.

> it is possible YC does not think we will be a billion dollar company

Most VCs always emphasize that they invest in the team and not just the idea. Ideas always evolve for startups. You guys not only monetized your MVP very quickly, but also have shown that you are hustlers and work very fast. Even if your idea "in current form" doesn't generate billion dollars, but you have high potential as a team to find a path to success.

I worked in early stage startup few years ago, their original idea didn't show huge success but they kept improving until they eventually found a path to huge success and then acquired by large company. The improvement all revolved around the same concept (which was an audio app)

IMO, the fact that YC didn't even invite you for the second round means that they need to really work on their selection process.