I have been waiting for a case like this. The boilerplate SV Software Engineer contract says that the employer basically owns your whole intellectual output. In practice it's extremely common for people to leave FAANG companies and start companies that are basically more agile "forks" of whatever they were working on before. I have seen this so many times at Google. Someone makes a cool internal tool/service, leaves, makes it into a SaaS company.
A famous example is Foursquare. Google bought Dodgeball, which was the original product. The founders got paid then left Google and ... started the same company again but called it Foursquare!
My assumption is these companies don't sue because they can't afford the negative impact on hiring. If you thought Apple would own your mind forever you'd never work there in the first place.