Calling an ex-President "President" is an honorific, a sign of respect for a former leader. ALL former Presidents used to be called like that until President Carter was defeated in 1980 and they began to refer to ex-Democrat Presidents by just their last name. Thus "President Carter" became "Carter" while ex-President Regan was referred to by the PROPER honorific "President Reagan"

Notice that former President Obama is often referred to simply as "Obama" (which is disrespectful to his former office), while Former President Trump is referred to as "the former President" or "Ex-President Trump" which is ALSO disrespectful. Larry King once said "You don't have to like the person, but have respect for the office."

If I met Donald Trump, I would greet him this way: "Good afternoon President Trump, it's an honor to meet you" I would do this as a sign of respect.