personal memory -> shared herd memory -> external persistent shared memory -> abstracted external shared memory.

Each is a superclass of the previous one, and the differentiator - as I said - is that once memory is externalised, face to face transmission is no longer required, and it also becomes possible to symbolically abstract, summarise, model and share experience without having to living it personally.

That's a difference in kind, not a difference in degree. It took humans a long time to invent it, but once it was invented it made a lot of other things possible, including brain tools like computers, which not only store information outside of individual experience, but can leverage innate intelligence in novel ways.