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The simple answer to this is that these days "functional programming" doesn't just mean the absence of side effects. It means strong type systems, algebraic data types, list comprehensions, etc. It is a distinct cultural stream in the development of programming languages. Of course "functional" has an original narrow meaning, but so do "Republican" and "Democrat".

When Rust introduced ADTs they were recognizably a concept from functional programming. It's a place or community of practice, not a purely descriptive adjective.