A language is not functional just because it supports higher order functions. Technically C even supports them (even though the ergonomics and safety of them are terrible). Would you call C a functional programming language? Obviously not. Rust is also not a functional language, even though it comes closer than most OO/imperative languages.

Kotlin and plenty of other OO languages have borrowed some ideas from functional languages in recent years because those ideas are useful. That doesn’t make them functional languages. If Kotlin were a functional language, then it wouldn’t need libraries like arrow to try to make doing FP in Kotlin even (kind of) possible.

Hallmarks of FP (beyond higher-order functions), in no particular order:

• Organization around functions as the fundamental unit of code
• Code primarily defined in terms of expressions and data transformations rather than statements manipulating object state (so languages that have big blocks of imperative statements like Kotlin don’t count)
• A general orientation around pure functions, even if they vary on the degree to which they enforce purity
• Explicit parameter passing being the standard and preferred way of providing data to functions, rather than methods operating on implicit state
• First class support for function composition (method chaining doesn’t count)
• Pattern matching and destructuring as a first-class and ubiquitous concept (what Kotlin does have is a joke comparatively and no one would actually call it that)
• For statically-typed functional languages, first class support for algebraic data types (Kotlin has sealed classes which can kind of be used to try to emulate it, but it’s pretty awkward in comparison and requires you to write very OO-ish code to use)

There are some minor exceptions, such as Clojure lacking pattern matching, but on the whole functional languages generally fit these descriptions.