While one can certainly over-engineer everything into N levels of abstraction either with OOP stuff or with FP stuff like higher-order functions (or higher-higher-...-order functions for that matter), I could well imagine, that a person who knows FP stuff would not have as much of a problem with a specific code base and when that person leaves and only OOP-only people are left, they scratch their heads and call it an "unreadable mess".

So I would not rule out that possibility, without having seen the actual code. Documentation is a point though. One should always document at least for a bit stupider version of oneself. One day in the future that self will come back and "not get it".

That said, it is at least possible, that the company in question needs to hire a smarter person, or simply a more FP informed person. Also entirely possible, that the code is over-engineered and way too complicated for what it achieves. Without seeing an example ...