Surely it's about functional application as much as it's about functional composition. E.g. currying, one of the signature idioms of functional programming, doesn't involve composition at all, but is more closely related to partial application.
I'm a bit skeptical of the premise that either composition or application is a high enough bar. Consider stack-based/concatenative languages such as Forth. Composition and application are foundational aspects of that language, but nobody really considers Forth to be functional. On the other hand, the concatenative language called Joy is considered functional -- by its authors at the very least -- but it also introduces purity to the mix.
Of course it's about function application. All of programming is about transforming data. This is obvious. Some procedure must be applied to some data to transform it. There is no programming paradigm or language that avoids this and therefore it is not worth discussing.
I bring up composition because this it is the most important differentiator between FP and all paradigms. I wrote about it another reply:
I'm a bit skeptical of the premise that either composition or application is a high enough bar. Consider stack-based/concatenative languages such as Forth. Composition and application are foundational aspects of that language, but nobody really considers Forth to be functional. On the other hand, the concatenative language called Joy is considered functional -- by its authors at the very least -- but it also introduces purity to the mix.