I don't know. While I don't dare to question MS, it does not sound that impossible to me. They have a record of being almost vindictive towards some specific offences and they don't like the system to be abused. Whether they may be able to recover the money is a different thing but if they could confiscate a part of cpf, then why not?
I doubt it could be always the case, but I would not be surprised if one day there is something in place that if you renounce your PR within X-years after you got it, then cpf = cpf-Y. Other scenarios also possible. Afterall it's an enterprise and in an enterprise you may be asked to pay back some money invested in you if you leave the company too early.