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.
Not all mods are like that, of course. My instance admins had to ask me like three times to be a moderator for one of their communities because I refused them multiple times. I only said yes because it was an unmoderated/undermoderated (at the time), low traffic community, and felt bad that I had refused so many times.
I used to be a forum admin for a gaming/programming forum with what I would say is high traffic (1000+ active concurrent users daily), and moderating that felt like a full-time job, and I had appointed like 10 other moderators to help. I don't have time for that no more lol.