Someone learning a first language isn't just learning a new language: they're learning how to program. It's a new profession, a new hobby, a new superpower.
The rest of the stuff (standard library, ecosystem, buildsystem and all kinds of intricacies) is just a mix of trivia and bureaucratic garbage you gotta fill your brain with but will all be replaced in 10 years anyway. Sure it takes time but it's nowhere near as important as actually knowing how to program.
Even changing paradigms (imperative to functional) isn't as hard as learning the first language.
x = 1
for i in [1, 2, 3] {
x = i
}
A lot of beginning programmers struggle with this kind of stuff, as did I. Reading this fluently and keeping track of what variables are set to takes quite a bit of practice.
I've been hired to work on languages I had no prior experience on, and while there was of course some ramp-up time and such, overall I managed pretty well because variables are variables, ifs are ifs, loops are loops, etc.
The argument that learning a second language is as difficult as learning the first doesn’t really hold water in practice as well, lots of people have done so.
I write elixir that smells like rails, but as the months move on, I change.
You also see it when people who've mostly done imperative programming tries their hands with a lisp or some ml based language. I've been there myself. You still find yourself using the equivalent of variable bindings, trying to write in an imperative rather than declarative style.
I guess when trying to learn a new language in a different paradigm you also need to unlearn a lot of the concepts from the former language.
IME a lot of the conceptual stuff transfers pretty well there too.