It sort of makes sense. If someone knows a language well, they shouldn't have much trouble writing it syntactically correctly on a whiteboard. Especially in languages which have simpler syntax, like Ruby vs eg Scala.
Interviews are often based more on what the interviewer knows than the project / resume.
Now what happens when someone asks about a language that you have not used in 3 years? Well it gets fuzzy. Ramping back up on an old language might take a few hours, but that’s meaningless in terms of a job.
The point is, it's much easier to focus on the idea of the algorithm when writing it down in pseudocode, without having to worry about c / c++ details that obfuscate the idea.