California's non-compete laws don't cover "trade secrets", only your ability to use your expertise to pursue employment in your chosen field. In other words, if you're just a regular programmer, you can go work wherever you like or start a competing company. If you're a principal scientist or architect, you would be in danger of violating your contract. Anyone who went with Altman would presumably have deep inside knowledge of OpenAI's secret sauce and therefore be restricted.
But there is an alternative scenario.. in order for Microsoft to avoid loosing any momentum they might offer Altman an insane amount of money to become the John Giannandrea of Microsoft and bring as many of his recent colleagues with him. And for Altman this might be the easiest way to not loose ground as well with Microsofts patents and license agreements.