This has come up a few times today. All presidents, past and present, are referred to as President. As someone else noted today, Jimmy Carter is still referred to as President Carter.

I believe Trump is our current, duly elected president and I'm still filled with hope that somehow or another this mess will be cleaned up or this "Plan" will come to fruition (though I am furious at how long it's taking). But the letter posted here isn't evidence of anything in particular. Even if Trump were to serve two more terms (he'd be a bit old by then), he would still be "President" Trump afterwards.