I dunno, there are plenty of slices of the programming world where you can quite profitably work in a single language for most or even all of your career. If you specialized in Java twenty years ago, you'd have skills that are still in demand today and will likely still be in demand twenty years from now (as long as you keep current with changes to the platform).
But betting your career on a single language is definitely a big bet. And any bettor will tell you one of the key skills is knowing when to cut your losses.
It's fine to happen to develop (or only have developed) in a single language - you might have found your niche and all that.
But the way the author of the article put it is just so weird - choosing the word "pure" in all caps - like it's a point of pride, or big part of their identity.
Java is a bit different though. Whatever we think of 'enterprisey', it does mean that a lot of systems were built with it with the expectation it would be around a long time.
As far as 'bets' go, it's also a bet on oneself. The skills of an 'average' developer between two languages can have notable differences in concepts and requirements. i.e. I'd be a bit curious about an 'Average' C/C++ Developer who couldn't write a linked list (or at least groks them), versus an 'Average' C# or Java developer who may never have to write one in their career.
I'll say that unless you intend to be a hardcore Systems developer or Finance Sector, Javascript is never a bad thing to at least learn enough of to at least pass equality questions on interviews.
The nice thing about learning other languages however is that it may help expose you to patterns you don't normally see in your main language, or may expose you to things that make it easier to understand certain concepts (i.e. doing Javascript sites with KnockoutJS helped me learn better MVVM practices, as well as concepts like observables which I could then use back in C#)
But betting your career on a single language is definitely a big bet. And any bettor will tell you one of the key skills is knowing when to cut your losses.