The problem with this claim is that it is not that easy to see it in the evidence. While nearly all the richest countries are democracies and nearly all the poorest countries are dictatorships, the causality could go the other way - or something else could have made countries both rich and democratic. If you look at growth rates you don't see big differences. You have to work quite hard to find differences (though some people think they can: https://www.journals.uchicago.edu/doi/pdfplus/10.1086/700936).