为什么不能使用import通过包实例访问函数?

我正在尝试使用ES6导入，并且遇到了vue-server-renderer的问题. 与此问题类似但不太一样.

I am trying to use ES6 imports and am running into a problem with vue-server-renderer. It is similar to this question but not quite the same.

运行此程序时出现错误消息TypeError: Cannot read property 'createBundleRenderer' of undefined:

I get an error saying TypeError: Cannot read property 'createBundleRenderer' of undefined when I run this:

import vueServerRenderer from 'vue-server-renderer'
const createRenderer = (serverBundle) =>
    vueServerRenderer.createBundleRenderer(serverBundle, {
        runInNewContext: false,
        template: fs.readFileSync(path.resolve(__dirname, indexFolder,'index.html'), 'utf-8')
    });

但是，如果我执行以下操作，则没有错误:

But if I do the following, then no error:

import {createBundleRenderer} from 'vue-server-renderer'
const createRenderer = (serverBundle) =>
    createBundleRenderer(serverBundle, {
        runInNewContext: false,
        template: fs.readFileSync(path.resolve(__dirname, indexFolder,'index.html'), 'utf-8')
    });

为什么必须直接从vue-server-renderer导入功能?通过vueServerRenderer.createBundleRenderer()而不是createBundleRenderer()通过vue-server-renderer实例访问它有什么问题?

Why do I have to import the function directly from vue-server-renderer? What is wrong with accessing it through an instance of vue-server-renderer using vueServerRenderer.createBundleRenderer() instead of just createBundleRenderer()?