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.
Then power-hungry moderators who behave like this can sully their reputation, risk the ire of the instance admin who may remove them over this, and if not - also risk the ire of the fediverse who might just recreate their community on another instance and supplant them.