I thought that idiomatic react involved using JS render functions to turn JSX + Data into HTML; and using css-in-js to handle styles?

I'm somewhat wary of React because I feel like it pushes developers to work with non-standard tools rather than the HTML + CSS + JS that browsers handle just fine.

I agree wholeheartedly on the ecosystem. I think global state stores will have a reckoning in the next 10 years where we collectively realize that maintaining a large amount of client-side state is not required for 90% of sites.