That guarantees the right to confront the witnesses in court, not necessarily the right to talk to them about the case outside of court. The last thing Trump needs is charges of witness tampering added to the mix.
Seems to me that when you have a right to all discovery and especially to exculpatory evidence under due process, that the only reasonable way to explore that is by questioning/testing the evidence (witnesses or whatever) PRIOR to a trial.
Otherwise, it would be a kangaroo setup.