There's nothing wrong with using Medium as a glorified pastebin; the OP does that by having Medium mirror content they post elsewhere, and it completely avoids the "assholes."
The problem only comes if you actually go in on the "game" of the platform—allow your writing to be led around by the incentive system inherent in the structure of the platform's network effects: recommendations, promoted posts, likes and shares, front-page status, etc.
If you "buy into" the platform like that, then the culture of the rest of the community on the platform (i.e. the other people who have "bought into" the platform) will start to matter. If they're mostly assholes, you'll have been tricked into playing a zero-sum game against assholes.
You know what a private Minecraft server with no moderation is like? It's an infinite expanse of dick-shaped monuments, where anything genuinely creative is quickly torn down or blown up. The parent poster is basically saying that Medium is the same way. The dick monuments are figurative, but they're still dick monuments; the tearing-down is social, but it's still tearing-down.
If you don't play the platform's game, none of this matters; you just get your little sandbox within the dickscape and can ignore the community around you. But the platform wants to engage your readers—through links back to a front page, or recommended follow-up posts, or whathaveyou. Building your sandbox in a dickscape leaves open the possibility that your readers might look out the window.