To be fair, the FBI is a big organization which employs many intelligent individuals. It's very unlikely we know everything about this situation that they do.
I would take everything they say with a grain of salt.
1. The FBI knows who did it and is telling the truth by saying it's North Korea. 2. The FBI knows who did it and is lying by saying it's North Korea.
Personally I think they're telling the truth in this particular case.
They explicitly mention that they are not sharing all of the evidence.
This is a habit that needs to change if they want greater support among the informed.