uibuilder provides a websocket to/from node-red, you build a website using bootstrap vue.
In the bootstrap vue documentation it is assumed that you run bootstrap-vue as a module of nodejs and use a compiler/bundler/build-step that will translate (transpile as they call it) into code that works in the browser, but that is not what you are doing with uibuilder - it runs directly in the browser. (note that you can use a build-step to make it work like that, but that is a different story).
Note the description of http-vue-loader:
Load .vue files directly from your html/js. No node.js environment, no build step.
This is the reason why you need to use module.exports
