Congress has not established any such body. And in addition, the VP is required regardless of which additional body is designated by Congress. But that's not the important thing, which is the context of the section.
This isn't about qualifications for office. This is what to do if the President isn't capable of fulfilling the office they hold and not able to resign. It's also worth pointing out that even a declaration under section 4 doesn't remove the President from office, just transfers the powers to the VP.
You don't have a point because the Trump case is about his fundamental qualification to hold the office, not the internal procedures once he is elected to the office. While it would never happen, in theory somebody in a coma can be elected to the presidency and there would be no Constitutional bar against it.
Only because SCOTUS decided that only the federal government could make that decision, but afaik, that wasn't settled law yet before this.