It's definitely not.
Source: I know a lot of people who had Caml classes before C classes, C was much easier. Also in my school we learned C and LISP at the same time, most people also found C easier.
I suspect that if someone has a strong background in mathematics they might take to functional programming easier than they might other languages. I could definitely be wrong though!
> many programmers don’t even have a firm grasp on recursion
I can’t say I ran into more than a few people like this and I don’t think any of them stayed in CS.
There are plenty of programmers out there that learned at boot camps, universities with poor CS programs, or who are self-taught, who likely have a much less solid understanding of CS fundamentals. Many of those programmers have never needed to understand recursion. I went to a school with a very average CS program, and many of my peers struggled with recursion. There were more than a few who passed the class (Data Structures) and graduated alongside me despite not fully grasping the concept of recursion.
Some of the best (functional) programmers I know learned (functional programming and programming in general) at boot camps. I think the predictor for FP proficiency is not education (and I hesitate to share my predicted predictors here). In any case, they certainly understand recursion.
One of the big reasons that Haskell is hard to learn is because it is purely functional. Writing idiomatic Haskell is dramatically different from traditional imperative C-like languages.
Just to give an example - a few years ago I was taking an AI class which used Common Lisp (a functional language, but not purely functional). The first few tests/projects which required functional lisp programs led many of my peers to drop the class.
I think part of this is because my school, like most, started students off with an imperative/procedural language (C++). If you have a clean slate (e.g. someone with zero programming knowledge) it’s probably just as easy to teach them Haskell as it is C++ or Java.
Functional programming is great, and I love it, but many programmers don’t even have a firm grasp on recursion which makes pure languages like Haskell like a mountain to climb.