It appears from the next chapter that young Alma had just been made president of the Church. His father was already beginning to feel that the end of his life was near and he had now reached the age of 83. Because of this development, Mosiah felt it was expedient for him to turn the Nephite records over to young Alma. However, as Mormon, the historian, began itemizing the records which would be given to Alma, he came to the 24 plates containing the history of the Jaredites and remembered that he had not yet told us that these plates had been translated by Mosiah. He therefore inserted these details sort of parenthetically and doesn't finish the sentence he had just begun until verse 20.
This is why verse 11 is so awkward. Mormon is writing under tremendous pressure and after he has painfully engraved something on the plates he doesn't have time to go back and make corrections in the proper narrative style. Consequently he just adds the correction and goes on from there. Today we have computers and it is a simple thing to make corrections. But Mormon didn't have that luxury. This is why he interrupts himself in the middle of this verse to tell about the 24 gold plates and doesn't finish this sentence until verse 20.