I think it’s a little more obvious than the other answers.

Trump did himself a disservice by telling people only to vote on Election Day - even if it’s better to vote in person, there could be someone who doesn’t have the means to get to a polling center or maybe they can’t vote for another reason. Maybe they die! Or maybe they just forget or get sick or something.

By telling people to vote early, you don’t let the democrats get a head start that we then have to overcome on Election Day. It makes no sense to tell people “don’t vote early or by mail in ballot” as you’re literally turning away voters. It may not be ideal, but they all count.