CSS is a different technology for a different complementary purpose to, say, markup or business logic. That's why we have the separation of concerns principle. If business logic is doing CSS's work, design changes MESS with the business logic, which is undesirable for obvious reasons.
By saying "CSS is more a separation of technology, than a separation of concerns" you are saying "CSS is more a separation of concerns, than a separation of concerns" or possibly "CSS is more a separation of technology, than a separation of technology".
As for the modularity concept, that doesn't have to flout or adhere to "separation of concerns". That's just a different thing.
I'm not familiar with React. I tried reading a primer on it and got really bored. Couldn't imagine how it would change my work. Then again, I'm mostly a UX designer who works with code rather than an "engineer" focused on abstraction and performance.
By saying "CSS is more a separation of technology, than a separation of concerns" you are saying "CSS is more a separation of concerns, than a separation of concerns" or possibly "CSS is more a separation of technology, than a separation of technology".
As for the modularity concept, that doesn't have to flout or adhere to "separation of concerns". That's just a different thing.