Look at the Native Libraries in the project in the link you posted. Their import works fine, e.g.:
<link href="https://releases.transloadit.com/uppy/v1.25.1/uppy.min.css" rel="stylesheet">
<script src="https://releases.transloadit.com/uppy/v1.25.1/uppy.min.js"></script>
in the Native Libraries, and then in the form you can use the from anvil.js.window import Uppy as shown in the post you linked to.