The issue is more a one of trust, as pretty much all computer security issues boil down to. Apple (and Gruber) says one thing; but since the code is not available for inspection, and since you presumably can't alter the code on your actual iPhone, you must take what they're saying at their word.
Whether or not you want to do that is entirely up to you. The overwhelming majority of people wouldn't think twice about trusting Apple. But, as has been has revealed, government influences are powerful and are already deeply (and secretly) entwined with private enterprise. To snidely dismiss people warning you about who to really, truly trust with things as fundamentally identifiable of you as your fingerprint is doing yourself, and them, a disservice in the post-Snowden era.
Whether or not you want to do that is entirely up to you. The overwhelming majority of people wouldn't think twice about trusting Apple. But, as has been has revealed, government influences are powerful and are already deeply (and secretly) entwined with private enterprise. To snidely dismiss people warning you about who to really, truly trust with things as fundamentally identifiable of you as your fingerprint is doing yourself, and them, a disservice in the post-Snowden era.