Honestly, your explanation of why he sounds guilty to you reminds me of the paranoid ramblings of, well, paranoid people. Again, it's entirely possible the FBI (though this isn't their jurisdiction really) could be trying to subtly shape free software to their bidding, but this is a baseless, and pretty shaky, claim by someone that allegedly has a commercial incentive in stirring up this shitstorm. I think the accused has every right to be mad as hell about the accusation, especially if he did no such thing.
In any case, I have no clue whether he had anything to do with any backdoor, just trying to make the point that his response is not what I'd expect from someone who had nothing to do with it -- possible exceptions: If he's an unusually egotistical person or if he has significant financial interests which the perception that he was involved could disrupt.
Or, if he is passionate about the software he is involved in building...which Open Source developers generally are. Reputation is the only currency that matters in the Open Source world, and someone has attempted to destroy this guys reputation. (Or any number of other reasons why someone might be bothered by such an accusation. Your assertion that there are only two "possible exceptions" is just ridiculous. Speaking in such certainties about the human brain and human emotions is simply nonsensical.)
Frankly, I think you're talking out of your ass here, with very little understanding of the people you're talking about, or the psychology you seem to believe you know so much about. Do you have no exposure at all to the Open Source community? That's the only way I can imagine you would consider reputation to be something a normal person doesn't have every right to care about and defend from accusations.
Honestly, this developer responded far more politely than I would have in similar circumstances.