Reliability is a quality that comes in degrees. For example, a bus that always arrives exactly on time is highly reliable. A bus that often but not always arrives on time is somewhat reliable. A bus that rarely arrives on time is unreliable. People living in areas with public transit commonly discuss which among the less-than-perfectly-reliable modes of transport available are more or less reliable. In doing so, these people show they understand that reliability comes in degrees. They readily acknowledge that some imperfect modes of transport are more reliable than others.
Propagandists prefer their audiences ignore this level of nuance when assessing sources of information. A propagandist prefers that you perceive the propagandist as totally reliable while perceiving all other sources of information as totally unreliable. If this cannot be achieved, the propagandist would prefer that you view all sources as completely unreliable. At least then your decisions about whose claims to trust will rest on grounds other than the reliability of the source.