The author begins with what he considers bad ways to include libraries in Vue, How not to include a library in a Vue.js project::
Global variable, the naive way to add a library to your project is to make it a global variable by attaching it to the window object. Importing in every file, second-rate method is to import the library into every file.
Then you can read about better ways to use external libraries into your project and further down the advice to build a library you’re planning to use across many Vue projects, into your own plugin.
Why not make it a plugin?