I think you're proving my point that Medium needs to clarify their brand. They are actually doing all the things you mentioned.
Here's what I can see them doing.
1. You can't get distributed by their algorithm if your article has blatant sales or a call to action.
This removes the incentives for a lot of content marketers. Medium literally has a human read and filter every article behind their paywall. That's thousands of articles every day. It's not very sophisticated, but it's good enough to push back on most of the marketing pieces.
2. Medium's promotion algorithm relies heavily on their Topic feature and topics are all manually curated by editors.
Topics, i.e. Programming, Data Science, etc, trigger most of Medium's distribution paths. They get an article promoted to people in the app and they get the article in the mix to be promoted over email.
3. Medium's Owned & Operated pubs (their term for their internally run ones) do very heavy story edits.
A copy edit is when you do light grammar and spelling and occasionally wording changes. But a story edit goes much deeper often taking several hours. Many of these articles are original commissioned pieces, but I know Medium often "elevates" (their term) an article from the general platform. Here's an example of how deeply an elevated article gets edited:
https://twitter.com/tonystubblebine/status/11533445070871633...
4. Medium is increasingly paying for copy editing
I run two publications that do copy edits. They are my copy editors, but are paid for by Medium. These pubs are community aggregators, i.e. they publish a lot. I think there's some value in this even though I know the highest quality articles require a lot more time and editing. Anyway, we copy edit about 500 articles a month.
5. Medium is experimenting with paying for external story editors.
I think my original pub, Better Humans, is the only partner publication they have that gets paid to do a story editor. We have someone with subject matter expertise spend 4-5 hours on each piece reading each sentence for "is this true?" and "could someone follow this advice?" And we do all the normal editing things like sending a piece back for revisions.
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In sum, I think Medium is trying to have it both ways.
One way is a high volume of "better than the rest of the internet" content. That's how my two new pubs operate. We just want to clean them up and filter out the worst ulterior marketing motives. But we are definitely volume first.
And then in a different dimension, they want to have prestige publications that feel must-read. I think this dimension is much harder, but it looks like they'll get there. (And I know they want to).