My theory on why Flutter is popular in developing countries is a Flutter app compiled in release mode is turned into AOT code. This gives it a performance boost on apps that are constrained by low CPU and memory resources.

I expect that that most HN users probably have a flagship mobile phone with lots of CPU, RAM and reliable mobile data options. The other 90% of the world can not say that and a lot more sensitive to apps that require advanced phones and operating environments to function.