Interesting ... but how could IBM's promise mean anything if TurboHercules, which is an open-source company, isn't protected against assertion of all 500 of those patents.
The loophole is a contractual issue between IBM, its customers, and perhaps TurboHercules (in that the latter is encouraging the customers to violate the contract).
I don't see it having anything to do with open-source patent assertions and again I can't see how IBM's promise would be worth anything if the latter had its own loophole of a "If we sue you for other reason the 500 patents become fair game."