I don't want to overreach here (given how little I'm working with), but it seems like you misunderstand the chain of causality. It sounds like the DGS was saying that your research experience in the subfield, in conjunction with your coursework (which obviously influenced your stated research interests) were key components in getting you admitted.
Stated research interests conditional on having actual research/coursework experience in the field of said interest is a big plus. It means you actually know what you're talking about because you have some first hand experience. It makes your SOP credible.
Stated research interests without having actual research experience in the field is well... "I want to become a development economist even though I've never taken a seminar on development or done any real research related to development". You can see why this might not be taken as seriously.
Its great that you got admitted to a school you really wanted to attend. I wish you luck.