I'm speaking metaphorically. When I say "import a second backend" I'm saying that the thing being imported into the ruby backend is literally a wrapper around the exact same runtime that is typically used to run js backends, the corollary being that the "ruby" part is unnecessary added complexity from an engineering perspective unless you have some other compelling reason to use ruby (which is certainly possible).
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...