Languages are simple syntactically, but I'd have a hard time believing that anyone could pick up the set of idioms necessary to effectively use a new language in a few days.
Coming from a strong background of similar languages? Sorry, you're wrong there. Perhaps you aren't familiar with Erlang; it is the "everyman" of functional languages. The things I think you'd call "idioms" (not really sure what you mean as that's a very loose term), let's say recursive programming, pattern matching, carry over as-is from any number of other functional languages. Pattern matching? It's a mix of OCaml and Prolog. Terms? Imagine Scheme had tuples in addition to lists. Etc. The only "new" concept is the message system, which if you know anything about networking, is pretty straightforward.
In other words, if you know pretty much any other functional language, it's almost trivial to translate basic programs to & from into Erlang knowing little more than its syntax, which as you concede, is simple.
OTOH, if you're coming from, say, PHP, sure, learning Erlang will be difficult as first you have to understand functional programming. But you're asserting impossibility, so counter-examples are moot.
Ten Years… I've been coding over twenty years in more languages than I can count… I have a good idea how long it takes to learn a language well. Maybe I'm biased because it's the latest language I've studied, but Erlang was the first non-toy language where I skimmed the reference manual, said "huh, that was all unsurprising", and started coding effectively.
Thanks for the link, it expresses part of what I struggle to explain quite well.
One problem I have, which Norvig doesn't talk about, is that I'm currently moving from one ecosystem (Microsoft) to another (the Java world), and it's not just about the language, it's everything, all the toolchain is different and unfamiliar.
Norvig it covers this well in his Teach Yourself Programming in Ten Years (http://norvig.com/21-days.html) essay.