I have to say that, even as someone fascinated with Scala this week, I find the argumentation here a little disingenuous. There are a lot of ways to filter developers and I don't think that the ability to learn a new language sets the bar high enough to actually select for the good ones and potentially means selecting the wrong tool for the job.
There are a large group of people that are effectively programming language hobbyists, frequently learning new programming languages for kicks, and while that set intersects the set of good developers, it is not a subset of it.
The initial hump in hiring is always the shortlist - you might get a hundred (or a thousand!) CVs and you can maybe first-interview 10 and second-interview 2-3. So the priority is always to get down to a manageable list by rejecting candidates who are obviously unsuitable.
So you are correct, but you could probably even trust your HR department armed with a suitable list of esoteric languages keywords (instead of their usual "Java" etc) to build you a pretty good first-interview shortlist, even if you were after Java developers.
There are a large group of people that are effectively programming language hobbyists, frequently learning new programming languages for kicks, and while that set intersects the set of good developers, it is not a subset of it.