Just what the heck is wrong with you?
Look, I’d normally expect some level of professionalism and language knowledge in the gamedev community.
Headers are frequently used to control visibility of portions of code as additional visibility control measure in additiona to access modifiers AND namespaces. The paper you linked gets the wrong idea. It assumes that “treating compilation units as separate” is bad thing. It is a good thing, it is actually frequently used feature, and I’d expect you to be already aware of that if you’re using C++. It is something you’re supposed to know and understand. Private/public/interface headers, specific to some feature headers, etc. With module based approach you’ll end up doing more typing.
And instead of working on better mechanism that can fully utilize resources of modern computer during compilation the article wants to modify language. Sure, who the heck would want to utilize gigabytes of available memory on modern machine.