Not joking at all. Functional languages have a reputation for being hard to learn mainly because they aren't what most people learn initially (and switching paradigms is never easy, no matter which direction you're going.)
Disclaimer: The above is an opinion. Double-blind trials have not been performed!
I agree that switching paradigms is not easy. The part that sounded like a joke and still sounds like a joke is downplaying the difficulty it is to learn OO abstractions and patterns. Even initially being taught them, does not mean you've really learned nor understood them well. When comparing it to something "elementary" I'd expect something simple like 13+4, not something as complicated as software design.
Nothing we do is easy for most people... that's why we're highly sought after.
What I meant was not that it's easy to do something, rather that it's something that often comes early, and is mandatory, in your journey of discovery. The concepts you learn early on, while not easy to fully grasp, stick, and make other paradigms seem alien.
Disclaimer: The above is an opinion. Double-blind trials have not been performed!