Perhaps this is a more useful answer: why do you believe that the developer of at least one text editor should be using Swift as an extension language? This apparently seems obvious enough to you that you felt motivated to ask the question, but it is not clear to me why it should be so. What might Swift's hidden virtues be, that are being overlooked?

Were I to add a scripting language into the editor I maintain, there are half a dozen candidates which quickly come to mind; if none proved suitable, I would probably end up designing a new language, tailored to my purpose, before it would occur to me to use some heavyweight, VM-driven beast like Swift (or C# or Java).