your understanding is incorrect
Actually, I think his understanding is correct, at least in part.
Since the donations are not being made to the Cruz campaign, Cruz cannot provide the donor information to the FEC. And if the donors to the super-PAC are 504(c)4 entities, they are legally able to shield the identity of the donors. So while the identity of the 504(c)4 group is known, the identity of the actual donors is not.
The folks at National Review seem to think otherwise though.