It’s not an assumption, but a conclusion based on the evidence. I’m not aware of any Asian, African, or middle eastern country where freedom and democracy led to prosperity. There’s lots of democracies in these places that haven’t been able to translate that into prosperity. On the flip side, China, Japan, Korea, and Singapore are all examples where authoritarian rule has led to prosperity. I’d include Bangladesh these days as an example of that also.

If the country you’re using as an example is in Eastern Europe, I think you may be overlooking key differences. Fareed Zakaria addresses this in his book “the Future of Freedom.” https://www.amazon.com/Future-Freedom-Illiberal-Democracy-Re.... He explains that many of the foundations of liberal democracy were put in place during autocratic regimes: https://en.wikipedia.org/wiki/The_Future_of_Freedom. Eastern Europe had many building blocks of democracy in place hundreds of years ago. For example https://en.wikipedia.org/wiki/Cossacks.