First, it's not a matter of just learning to do something and moving on. There is incremental mental baggage that comes with every one of these "tricks". Since we are both Javascript developers, I won't list any, but I think we both know that some of the things you need to remember to do go beyond adding an additional equals sign. The sum of this is you can often find yourself falling into one of these traps, even after many years of experience, because there are just so many places you can be tripped up.

Second, I'm going to disagree with you on having a polygot codebase. If the developers decided that CoffeeScript is a better environment than Javascript (which you may disagree with), then I think it's clear the ideal scenario for them is their entire codebase is in a single language. Since it was tractable for them to convert the existing code, it seems logical to do that (not withstanding concerns like version control history, etc, which seems like an academic concern not really that important in practice.)

In fact, converting an existing code base is a common way to learn CoffeeScript and come to a judgement if it is suitable for future development. Such seems to have been the case here.