That's news to me! Does ITU have any ability to mandate laws for the physical spectrum? If not, a global communications committee would still be needed. At minimum, they would need the ability to control the 5G blocks of every country participating in the SAT to cell service.
Like most of these international committees, I believe they mostly facilitate agreements between sovereign states; I don't believe they have any legislative power themselves.
Maybe an analogy would be ICAO's (another "specialized agency of the UN") "Freedoms of the Air" [1] – they define the terminology, but it's still up to individual (or blocs of) nation states to actually grant them to each other, ratify recommendations etc.
