> The clipboard should not, nor should applications be able to, alter the copied content from the visibly-selected content.
This might feel intuitively right, but it severly limits the usefulness of the clipboard.
It then becomes a basic plain text clipboard
Try opening an rich text editor (e.g. https://quilljs.com/playground/) and selecting two words of which one is bold. Hit Ctrl+C. What is now on the clipboard? What happens when you paste? You have your formatting preserved. The editor has intercepted the copy command and stored data only it knows how to create and parse. The same would happen in a diagram editor (a selected shape would perhaps be stored as some json representation), or an image editor.
The problem is that the clipboard as an established concept already means "area where programs write their custom formatted data and where any app can read the same data upon paste".
Restricting it might be a good idea in some cases, but it's the user expectation so it's what apps (including browsers) need to do as the default. This unfortunately means that abusing it as in the article will be possible. There is no way for the browser to know whether the changed content was formatting tags (good) or trashing the selection by adding a copyright (bad).