There are so many wrong saying in this statement.
First, understand that CoffeeScript is JavaScript - but without its idiosyncrasies. If you know JavaScript really well, learning coffeescript takes minutes. CoffeeScript embraces closures and prototype inheritence; it just makes it less cumbersome to use.
Second, who said that programming in CoffeeScript means you don't really know JavaScript? CoffeeScript let you write better code in less line. (By better I mean easier to maintain and read). Furthermore, CoffeeScript programmers tend to choose it because they enjoy coding in Javascript and want to keep using it. What is way more scary is maintaining codes of designers who use jQuery without understanding Javascript.
Third, I hate the "For the extra effort" part. It's like saying, either you learn Python or C++; you can't learn both so better spend the time learning C++. It's just totally wrong. Good programmers tend to enjoy learning new technologies and paradigms. I'd even go further by saying that learning CoffeeScript will make you a better JavaScript programmer.