Except that Tolkien began the Silmarillion in 1916-17, and it was very close to the 1977 "final" form in 1937, when the first edition of The Hobbit was published. In fact, he offered it for publication in 1937, and it was rejected. Although he continued to work on it until his death, it was a fairly complete story before he went on to write Lord of the Rings. The bits of the Silm that were changed, such as Celeborn's origin or the Second Prophecy of Mandos, don't really introduce any errors into LotR. So comparing the Silm to the SW prequels, which were written decades after the "later" material, is not really accurate.