I just read it again and it's true there's a lot of marketing speak, but it seems you've been blinded by it and ignored all of the rest.
It's great to have these questions and insightful answers. The thing is you've said your friend is nuts about it, but apparently haven't bothered to ask them the same questions you're asking here. Not only that, you do seem to be fairly interested in knowing what it's about, but you don't seem to have read anything about it at all, so maybe you could have at least followed the hacker spirit and... I don't know, scanned for the links on the top of the page... and formed a basic opinion?
https://www.notion.so/wikis (I've just seen you can even try a live demo without signing up)
https://www.notion.so/projects
(There are more, but I believe you get the idea)