Unable to import a module from outside the project directory

I have a case where I’d like to import a file that is one level up from the project directory. An example is like this:

… /myApp/in-directory.helper.ts

Where external.helper.ts is something I’ve written. There are zero TypeScript errors during compile, but when I run it in the browser, the external helper is undefined (this is imported in ./myApp/app/app.ts). See example repo: https://github.com/astopo/ionic-import-out-of-project-directory

I’ve tried including the external.helper.ts to the filesGlob in tsconfig.json, but that doesn’t seem to resolve it. Any ideas?