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.
It used to be that the best posts would have hundreds of upvotes and hundreds of down votes. They showed +100/-98 and you did mediately knew this was an interesting comment.
Then they stopped showing both up and down, and only showed the summation. 100 upvotes and 98 down votes is now +2, and this comment is now lurking among all the other +2 comments.
Showing the total instead of the ratio was the end of reddiquette, and the earliest Reddit enshittification that I can point to.