There must be something we all missed - apparently the M7 chip does protect against Heartbleed.
see here : ADI Demo
That page isn't working, at least for me, but I doubt it shows that SSM would prevent Heartbleed in every case. There are a huge number of possible permutations. And a huge number of targets for attackers to try, so that 1-in-16 would still have left an unacceptably high hit rate.
Also, it's much easier for something like this to catch something like Heartbleed if the attacker is aggressive and opens the DTLS heartbeat window as wide as possible (i.e., tries to grab all 64KB). A careful attacker might gradually widen it, hoping to get something useful before triggering a trap. That's easy to automate.
Of course, the article says Larry claimed "SSM would have discovered Heartbleed". Discovering is not exactly the same thing as preventing.