I still don't understand the UX. Am I assuming correctly that it's core to the concept of Notion to always be editable and not have an edit/view flip switch? Because I never used Notion to write anything, I just read things other people wrote. And so it's arguably weird to interact with a webpage that as it turns out is secretly an editor only masquerading as a webpage. Sly!

Btw, if links inside a contenteditable aren't supposed to behave as links, why make it clickable in the first place? Because if you make it act like a link, you indeed have to reimplement the whole thing because of expectations.

I'm no interaction designer, but imho a simple edit button would be mucher simpler. It could still be integrated nicely, no need to have wiki-like editing where the entire context is lost.