i don't get how it's an argument for encryption if that wasn't the affected part
The government demonstrated that they are unqualified to keep data securely under their control. As such, any argument that a backdoor only allows the duly elected government access is empirically false. Any such backdoor empirically allows nefarious criminals access to your messages whereas the absence of a backdoor keeps your messages safely between you and the other party.
So, your options are:
1. Encryption with no backdoors so all messages are safe.
2. Criminals can freely read your messages so the government can more easily investigate potential criminals.