I'm not sure this is a fair criticism. The nice thing about this tiny chunk of code is that there is only one aspect of this example program that needs to manipulate the already instantiated DOM, and that tiny chunk is explicitly brought up to the top level and separated and connected back to the program via the port mechanism. It would certainly be worse if this magic happen behind the scenes.

Of course it could be better, and that's probably what you are getting at. Ideally the machinery provided for building web applications would be more aware of the nature of web applications and have something in the box (still explicit and obvious, but in the box) to do something like setting focus.