There's a lot of progress that can be made in cross-disciplinary areas, but if it's done by one person it tends to be someone who specializes in one thing and stumble upon a low hanging fruit elsewhere. Much more often, in case of deep results, is that two very smart people in their areas talk about their work and discover an opportunity to collaborate.

Most people who claim to be polymaths are dilettantes. Even if they are really smart. People who claim they're polymaths don't have the humility to recognize it's very difficult to be at the forefront of two disciplines. A dilettante wouldn't know this because they don't know the amount of effort and learning it takes to be a true expert.

A true expert spends a lot of time on a subject and may on the surface may appear to have no more knowledge than someone who just reads and follows their papers for less than half the overall effort. This is because much of extra work is spent on trying different ideas and learning for producing the next good idea, or trying to apply it effectively in various contexts, rather than just learning other's good ideas for the sake of it.

Whether the expert is more valuable in dollar terms or potential impact than the dilettante or a humble generalist is another question. People can indeed be good at wearing multiple hats and create value. But I think someone like that should recognize that they are trading depth for breadth and not claim they are better than others at everything they learn.