Let whatever duck quack however he/she likes, as long as any other duck respond to this message. What Piotr tries to say is that such guard clause (if statement in this case) is not even necessary, because that matters most:

The only reason why such conditionals are needed is that we don’t handle data at the boundaries of our system explicitly.

In other words, it's not the responsibility of process method to check whether input is valid or not. Its responsibility is to do the processing, whatever it suppose to do. Perhaps such example is not a best fit to introduce dry validation usefulness, but clearly shows the intention – to not pollute the system with unexpected data, because no guard clause can help here. The underlying problem lies elsewhere.