I think the idea of a backdoor comes because CIA/FBI/NSA has incentives to be anti-encryption. It is their job to break it. So they want their lives to be easier. But the DoD has an incentive to have strong encryption. It is their job to defend their communications.

But I'll disagree with you (while agreeing with you) on this

> Everyone here is speaking like there HAS to be a backdoor

The DoD __does__ have a backdoor. It is the cellphone, not Signal. There doesn't have to be a backdoor in Signal for them to have full access to these communications.