So imagine a tiny cluster of top 20 witnesses decided to get together and run code that claimed a block was irreversible, but then were like "just kidding" a few blocks later and voted to remove the block?

This is something new for me. I thought that every block is irreversible, and that this makes this platform resistant to censorship, because nothing is editable on the blockchain level (every post/comment edit/delete is visible on the blockchain level). But even if this (block removal) would be technically possible, I still do not see the reason, nor the goal behind it. So why would they do such a thing? For what? The only reason I see is deleting some kind of illegal content. But would not that (removing a block) break the integrity of the blockchain?