There is a thing called web-development, emerged in the last couple of decades. It's whole purpose is to develop apps using this hypertext document rendering engine. You're not wrong by saying that browsers' main job is still rendering HTML documents. But most of the web pages that people use day to day are not "documents", they are apps.
Here are tabs I have open at the moment:
- HN (board app, not a document)
- Soma.fm DefCon radion (music streaming app, not a document)
- Facebook (social network app, not a document)
- Eurosport Player (video player, not a document)
- research paper (PDF, document, but not HTML)
- another forum (board app, not a document)
- Canva (graphics editing app, not a document)
- Miro (whiteboard app, not a document)
So you're disagreeing that those apps are apps and not the documents?