> why just using map/filter/reduce and avoiding state is enough for code to be called "functional programming"

A better term would be "programming with functions as first class citizens", but many years ago that got shorted to "functional programming".

> Most functional languages feature pattern matching, algebraic data types, purity, currying, strong typing, type inference, recursion instead of loops and types that cannot be null.

That's a list of what many functional languages now contain, although most don't contain all of those. But that isn't the list of "common functional programming language features" from 10 years ago, and isn't necessarily what the list will be in 10 more years, either.