This isn't clear to me, either, and it also seems from the docs like images, videos, and formulas are the only embeddable content types. However, elsewhere they say "Quill exposes its own document model, a powerful abstraction over the DOM, allowing for extension and customization. The upper limit on the formats and content Quill can support is unlimited. Users have already used it to add embedded slide decks, interactive checklists, and 3D models."

I wish they had a page covering what it takes to create a custom content type for the Quill DOM.