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.
Perhaps for some posts / comments. But definitely not for all of them. Votes can often be more useful than just feel good or feel bad. Very busy posts often have hundreds of comments. While certainly silly memes and the like may get upvoted there, often relevant or helpful comments do too, with unhelpful or toxic comments generally getting downvoted. Without that system in place I would have to scroll through those hundreds of comments just to find relevant or helpful info instead of not being at the top because the community provided feedback.