Fredo praying too loud may be dismissed as a "device", but the whole film is a "device". There's a difference between the presentation of the events in GF2 as "fact" (in which case logical arguments can be used) and the presentation of events as art, or symbolism.

In Oedipus Rex, I think Oedipus always "knows" the truth, because of the odd guilt in his statements throughout the play. This is not literal knowledge in the sense of fact, but symbolic knowledge - foreshadowing.

The hints that Fredo "knows" his fate is like this; it isn't knowledge in the logical sense, but only as a foreshadowing device.