The fact Sauron could delude himself this way means that he had/obtained a very wrong idea of Eru. Not only Eru's motives and attitudes but of Eru's nature. He thought about Eru as if the latter was somewhat a first among Ainur, not an ultramundane spirit, having all of the universe, Ainur including, existing in his mind.
Therefore Sauron perceived the music as an outline rather than something that had decided Ea's destiny. That were the prerequisites for Sauron's delusion, esp. his idea that Eru can abandon Ea.
Well, it seems at some point Sauron decided he had a better understanding of matters than Melkor. The question is when. If it happened before Melkor's final downfall, it is unclear why than Sauron did not abandon Melkor.
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