The first time I read your post I missed the parenthetical in the first paragraph, and was inclined to agree with you. Then I looked again and saw "(late twenties)", and had to laugh.
On the one hand, you're right, you can't learn everything. On the other, to be able to realistically claim that you can pick up new languages quickly, you should already know three or four different kinds of languages. If all you know is Java, you probably are farther away than you think from picking up a very different language like Ruby. I would encourage you to do some study on the side. Your late twenties is far too early to let yourself get pigeonholed.
Sure I know other languages: Shell Scripting, Perl, HTML/CSS/JS, Groovy, SQL, C#, etc, but 80 percent of the code I write is in Java. I'm at a Java shop and have golden handcuffs. It's Java or bust for me.
Don't doubt yourself. There are a lot of great companies hiring Java developers; you don't have to worry yourself about the standard YCombinator group.
I found it difficult to pick up Clojure and Haskell "in a few days" when all I had was experience in those types of languages. In fact, mastering each of those languages requires a change in thinking that can only occur over a much longer time.
I did do some non trivial work in Lisp in a graduate class, but its not the same as using it all day. I don't believe I would seek out an opportunity where a Lisp dialect was the programming language of choice, anyway...
(I found ML to be easier to work with than Lisp. I guess it's technically not a pure functional language though).
I'm also working mostly in Java recently, and our project has a small amount of Scala (left over form and engineer long since gone). I am finding it really hard, on the rare occasions when I need to work on the Scala part, to get my head around it. I know I need a week or two to "get" Scala - but I just can't justify the time.
The parent could be stating that late twenties is the average age of startup founders, not that that they are in their late twenties themselves.
I do agree with you that someone who claims to pick up new languages in a week should already have picked up a few languages. The grandparent says they only do the _majority_ of their programming in Java, so perhaps they have that covered. I'd love to know what the other languages they've chosen are, and their dissimilarity to Java.
I don't know how to evaluate their claim that they're a really_good programmer, given that it's a throwaway account. But then, given the data of this article, I guess no one does.
On the one hand, you're right, you can't learn everything. On the other, to be able to realistically claim that you can pick up new languages quickly, you should already know three or four different kinds of languages. If all you know is Java, you probably are farther away than you think from picking up a very different language like Ruby. I would encourage you to do some study on the side. Your late twenties is far too early to let yourself get pigeonholed.