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.
the implementation that piefed used to use made it trivial to link them to the original users, yes. this was an implementation flaw that could easily be addressed, which would make it less trivial to do so, mostly turning it into a probability assessment when correlating with other activity, provided that the pseudonymous identity is permanently tied to the real user.
The next step is for people who don’t want anonymous proxy voting in their community to simply ban every proxy voting account. This is the approach dubvee.org used.