According to law if there is no such clause the developer may claim ownership of code he wrote even if he was employed. The contract protects the company from that but also applies to any code written by employees generally (not a nice thing but prevalent).
Google, on the other hand, seems to believe that absolutely everything its employees ever do is relevant to its business.
Indeed, the main dev has forked it into his private Github account: https://github.com/echeran/clojure-turtle
Given how many reply to the approval emails that they found it fast and happy, i definitely find this surprising.
In any case, you are welcome to have those employees reach out to me, i'd love to understand their experience what went wrong.