Since when has that stopped companies and countries acting in their own best interests?
SpaceX has launched thousands of satellites, and plans to launch many thousands more, polluting sky and space observation for everyone on Earth. This happened relatively quickly, and doesn't require approval from every country, unless they intend to offer service there.
Similarly, it's not far-fetched to imagine a scenario where a "benevolent" billionaire, as the article puts it, or a single country, could decide that SRM is a good idea, and just go with it. Countries still pump excessive amounts of CO2 into the atmosphere, and historically can't align on a single policy. Why would something like SRM be handled differently?