But you can't prove it either than NSA doesn't have backdoor access to your fingerprint. We'd just have to believe Tim's word for it and they already lied about it once in the past. You also can't prove the fingerprint reader isn't hackable.
So the safest option is not to use fingerprint scanners.
Most people have their fingerprints in government databases anyway, they wouldn't want to go through the trouble to get them off of iPhones when they can get them easier elsewhere if they need/want them.