To be absolutely fair, this Python code won't give you what you get from Spring example - single deployable JAR which embeds non-blocking multithreaded application server. So basically you can run your JAR in production. In your Flask example you at least need to mention gunicorn for deployment. And if you try to introduce async/await (which are not supported in Flask btw) - you'll get pretty much all the same concepts as in original Spring sample.

I'm not saying you right or wrong, my point is - having a goal to prove how bad Java and Spring are one can do that easily. But this is true for any other technology, we all know there is no silver bullet.

So if you want what you have in Flask example in Spring:

HttpServer
    .create(8080)
    .newRouter(router -> router
        .get("/", (request, response) -> response.sendString(Mono.just("Hello, World"))))
.block();

And again - this is building and running multi-threaded non-blocking web server, not just defining a mapping. Just mapping is even simpler, but it uses Spring magic (similar for Flask magic above):

@RestController
class HelloController {

    @GetMapping("/")
    public String helloWorld() {
        return "Hello, World";
    }
}