What is TiddlyWiki good for?
The key points were that Wikipedia style wikis force you into a page-at-a-time model. And TiddlyWiki uses more like a post-it-note sized amount of information. Wikipedia has a few links, TiddlyWiki items would have dense links to other notes. Then the experience of reading something would be like a 'choose your own adventure' book - click on a link and you don't get whisked away to a new page with all the context lost, you get the next note appearing below. It's building on the work of Ted Nelson and the early HyperText dreams from before the Web.
As you click and explore, you build up a longer page of just the ideas and content you want, and don't have to write huge topic-wide essays and browse huge topic-wide pages all in one go. He called it "the correct level of granularity, deeply intertwingled".
There were probably other important points I've forgotten.
Relevantly for Dendron, the slide I've clicked onto in the YouTube has a quote: "Hierarchical and sequential structures are usually forced and artificial. People keep pretending they can make things hierarchical, categorizable and sequential when they can't" - Ted Nelson, 1974.
If you're trying to create a model of the real world, you probably want to use a graph primitive to capture the nuances. But that flexibility is what makes things hard for humans - since a note can exist in N places, how do you find it? How do you even know you've tagged/linked it correctly?
Dendron's constraint of hierarchy is one of those "its not a bug but a feature" things - the constraint of hierarchy makes it easy for humans to reason about. What we add on top of that is the ability to refactor that hierarchy when necessary