I think you're approaching the quote the wrong way. It's more like "a C++ developer should be able to look at Javascript and ascertain the gist of what's happening, even if they can't finely dissect the exact issues on a language level."

Though the quote is more broad than that. It's really saying "a C++ developer should be able to write up a report, participate in an interview panel, plan a small party for Jane's birthday this Friday, and be able to make conversation with the external partner coming in today". Few of us are simply just writing code all day.