Dorico is indeed better, and definitely not a copy of Sibelius UI-wise. Not only was the team starting over with a lot more experience under their belt, but they also had a much better idea of what didn't scale (from a user interface perspective) from the original Sibelius to its current incarnation, which has tons more features.

It's worth pointing out that when Sibelius was first released, it was hugely nicer to use than the industry-leading product at the time (Finale).