I was confused too, at first. I looked it up, and apparently it's just a sign of respect. If you were president once, you can still be called "Mr. President" as well as "former president." Feels weird to me, but hey, THE MORE YOU KNOW. 🌠
Why did he complain about ABC after the debate? After all, they kept addressing him as "President Trump."; Thank goodness Harris corrected it by calling him "Former President."