Only a few years ago, I would never have believed this could be real.
This doesn't strike you as exactly the sort of thing that would be created to make the left look crazy and violent?
Think about it - if there were such people planning to "attack" should Trump not concede, why would they send this warning letter? To tell the recipient to make sure their insurance policies are updated? So kind of them. They don't even tell the recipient to do anything re: the election - to vote for biden, to somehow get trump to concede, etc. The supposed sender, assuming they are anti-trump, has nothing to gain from sending this letter weeks before the election.
However, if the sender is pro-trump, they have something to gain from fear-mongering and maligning the biden camp.