ImGui makes UI programming declarative in structure the exact same way React makes UI programming declarative.
React brings to the table is the general concept of immediate mode UI programming. So does ImGUI. Reaching for React doesn't mean it's the only thing the programmers knew, it means it's a general pattern that arises in UI programming.
Sorry, but they are the same concept. They’re both immediate mode GUI tools that provide a way to declaratively describe the UI in an imperative language.
Let me introduce you to https://github.com/ocornut/imgui