Advocates of a certain language or a certain paradigm sometimes claim that adherence automatically yields clean code. Sadly, this is never true. You can make a mess in any language or paradigm. Only discipline, care, and attention to detail can stave off the mess.
I think a lot of people who say that are only thinking of their own experience - "when I code with this paradigm it makes me mindful of pitfalls that hinder me in producing clean code so I code cleanly"