I don't see why anybody believes the FBI aren't just lying at this point. They pretty clearly lied or at least obfuscated the truth on several other points so far. It seems obvious to me they never cared that much about what was on the phone and were only after a legal precedent - this claim about being able to unlock it another way is just a way to save face. It will be interesting to see whether they ever make public any evidence they claim was gathered from the phone or whether they just quietly let it slide into history and hope everyone forgets about it - I'm betting the latter.