Their point is that a lot of the underpinnings of the internet are based on not assuming safe delivery, nor assuming the sender will get things right, and generally accepting that if you want to interoperate reliably you will have a very hard time if you assume everything works perfectly.

But taking it a point further: A huge amount of large websites are built on languages with this level of dynamic features, and often take advantage of it heavily. Every site that uses Rails for example (or any Ruby framework) will likely heavily rely on runtime introspection of the database to dynamically generate parts of the ORM apis.

I'm right now working on a Ruby application where I went a step further and axed a large chunk of semi-manual API writing by writing a few small components that dynamically generates most of the API at runtime from small pieces of metadata and the actual state of the database. A large chunk of the userinterface is similarly dynamically generated from data dynamically generated by the dynamically generated backend API.

There is no way we could deliver what we deliver with the kind of size team we're using if we could not rely on introspection and dynamic features like this to generate most things.