There is no shortage of tax havens. Anyone who wants one will just move to the Bahamas. They already do, as you point out. Charter cities are thus totally orthogonal to this (despite the article's misleading implications).
The point is to improve life for the 99% of the world who can't just move to a tax haven, by building prosperous cities they can move to. This is not just a theory - China lifted 500,000,000 out of poverty using Special Economic Zones to transform their economy over decades. Many countries aspire to the same advance in prosperity for their people.