ExecJS functions as a JS backend in this case, its just a ruby interface for node/rhino/v8 that's a little bit clunkier to utilize.
This is not typically how the term "backend" is used in this context. Most mature apps will use a variety of tools and services built in a variety of languages, but this is not referred to as "importing a second backend." In fact, your comment is apparently the first use of the phrase "import a second backend" on the entire internet: https://www.google.com/search?q=%22import+a+second+backend%2...