Trump - never avoid a reason to insult someone.
He could have just welcomed Tim to the race. That would have been great, and enough. Trump being Trump could not leave it at that, he had to try to hit DeSantis? Why? DeSantis is a more formidable opponent than Tim Scott. Trump is not concerned about Tim Scott. Were he concerned he would have lobbed some snide remark about Scott.
Trump treats politics as a contact sport. He's competitive.
It's not in his nature to be sweet and gentlemanly. He's from Queens after all, not Antebellum Atlanta.
Fear and desperation will do that to people.
One can only imagine what he calls Tim Scott when the doors are closed.