I'm just curious how the founders were able to somehow negotiate a deal to sell Siri and then go right back into the same arena and create a "better" version. How would apple or any company allow those terms?!?
They didn't just sell to Apple and immediately quit. They stayed around for some years, which is a common contract condition. You can't practically require people to not innovate after they left a company.
Non-compete agreements are signed in (many) deals but they would be for a fixed term (typically 1 to 3 years?) after which, the inventors/innovators would be free to carry on further.