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The mod banning these users is the same mod who made the posts they downvoted. This is mod abuse, turning the downvote button into an auto-self-ban button.
The message is "If you disagree with me, you will be banned"
Monitoring and banning users for using lemmy as intended to signal boost your opinion should be grounds to have all mod privileges removed. This behaviour undermines the integrity of the server and the wider fediverse.
The sine qua non of censorship is the authoritarian component missing from your definition. A moderator or administrator removing an article from a forum is censorship. A user demonstrating their disapproval of that article within the forum doesn't qualify as censorship.
The Oxford definition is not wrong, just incomplete.