When talking about the “unbroken belief” I think that Steyerl is talking about the inherent trust that we have when we watch documentaries. The way that documentaries are portrayed and marketed to us we are lead to believe that whatever a documentary says is factual, and that a lot of these documentaries are uncovering massive secrets that governments and big companies don’t want us to know. Now in more recent years there have been an increase in the amount of misleading documentaries that have been released, whether they twist the words of other people, or just straight up lie, but people believe them because we have been told that documentaries are the most trustworthy films we can watch. This is the belief that misleading documentaries rely on to trick people into believing absolute nonsense. This is an example of how fiction can inform the truth because if we are being shown a fictional story but in the style of media that were are told to trust this may twist what we believe to be true.
The ways in which the truth is portrayed to us largely determines our view of the world, if we are being told everyday that something is the absolute truth if it turns out that is false we will have a tough time changing and thinking of the actual truth. Also if something is being represented in a certain way then that is how we will always perceive that thing. For example if we are told everyday that something is bad then that will be the truth to us and we will think that anything else is false.