just to keep the place from falling to complete shit, is it really decentralized?

Yes, that's the point. There's game theory at play here. That's called decentralized governance.

Sounds to me like there's a small group of people deciding what's acceptable and what's not. A bit like vigilantes.

It's small because most people don't get involved with governance and fighting abuse.

Let me guess, most of these people have massive amounts of STEEM, in other words, are untouchable whales and insiders?

Nobody is untouchable, ask haejin.

It's just the stuff that clearly comes from out of nowhere and you just say it like it's a foregone conclusion, like "The only way to fight this is...

I am one stakeholder getting involved with governance stating my opinion, as is my right. You can disagree, and have your own opinion that's fine.

why is it not mentioned anywhere in the official descriptions of this site/chain? Not mentioned in the white paper, not mentioned in any rules, or guidelines, or anything official?

Here we again, another person appealing for centralized governance. Do I need to explain again that the governance is decentralized? People will disagree and express it with votes. If that is too messy for you, too bad, downvotes are a central feature of this blockchain.