But it is defined in the contract.Sounds more like a recipe to get the OGL deemed unenforceable by a judge. Product Identity is not an actual legal concept.
Anway, it isn't important. My point is just this: Open Gaming as not just a way to make money but an actually philosophical perspective and active artistic endeavor is worth the trouble, and having an unimpeachable document in support of that has value -- even if "technically legally" it isn't necessary. Moreover, it is worth abandoning WotC entirely from an Open Gaming perspective and never again letting them define the terms.