Sounds like he may have inadvertently said the quiet part out loud... that he was Trump's doctor.

I haven't researched what the guidelines are regarding the medical treatment of the acting President of the United States, but I'd assume there are certainly guidelines in place, and not just any doctor who isn't officially approved/vetted/certified can treat the President.

Last year "The Zelenko Protocol" was known to be the best option for Covid patients. Maybe Trump's team used his treatment, or at least consulted with him, but couldn't publicly acknowledge that, due to guidelines or optics. Just speculating here.