I’m also personally quite confused about the distinction between Article, Document, Note and Page. They don’t seem to have any meaningful difference aside from the type name. It’d be great if this was consolidated into a single Post or something like that.
I also find it weird that Link is not a subtype of object.
On another note, I think in general inheritance is just bad and the Activity Vocabulary is built around an inheritance hierarchy. It should instead define each type independently and then say what behaviors are expected from each type. This is kind of analogous to hardcore object-oriented programming (such as in C++/Java) versus a type-class/trait system (such as in Rust/Haskell).