I publish Medium/writing guides. But it was driven by my desire to slow down people charging for writing tips (stealing their thunder by making even better content available for free). While I have published a lot of Medium specific content, it is not because I find curation difficult. Over 80% of my non Medium related articles are curated (obviously the Medium related article articles are neither eligible nor intended for curation). This is at least 50 articles. Not to mention, I have 6 other non Medium related publications.
So while I agree with your first point (Medium is promoting their own internal publications, perhaps to a fault), I would argue there is a bit more nuance to the second point about the clearly delineated tiers of writers.