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.
This is something that would definitely be better to message the instance admins directly over, and not turn into a public Tomato Toss. I don’t know the specifics, but there could be a reason for this (potentially Anti-Vote Brigading). When in doubt, contact the instance admins, and if they don’t respond or don’t do anything, then create a new account on a different home instance. That is the point of the fediverse.