Trump obviously has a more instinctive sense of how to motivate people than this demonstrates although the notice itself didn’t look all that official. How a semi engaged voter can believe there is such thing as a formal “violation” for not going to the polls I’m not quite sure.
Trump’s letter is one I would expect from a political campaign: Hi, if you are a Republican voter please vote for me and this is how you can go about doing that.
That is perfectly legal. No threats, no intimidation, no nothing; if you are going to vote then this is how.