Why does the Genesis Apocryphon from the Dead Sea Scrolls (like the Book of Abraham) claim that God is the one who warns Abraham that the Egyptians will want to kill him to get his wife?

In Abraham 2:22-23 the Lord warns Abraham:

And it came to pass when I was come near to enter into Egypt, the Lord said unto me: Behold, Sarai, thy wife, is a very fair woman to look upon;
Therefore it shall come to pass, when the Egyptians shall see her, they will say—She is his wife; and they will kill you, but they will save her alive; therefore see that ye do on this wise:

The idea that God told Abraham this is not found in the Bible, so where did Joseph get it from?

Jeff Lindsay remarks:

The Genesis Apocryphon from the Dead Sea Scrolls (pp. 26-29 in Tvedtnes et al.) indicates that Abraham had a dream which warned him of the threat to his life in Egypt, and of the need to have his wife claim to be his sister (p. 27).
– Jeff Lindsay – Questions about the Book of Abraham, Part 3: Does It Agree with other Ancient Texts?

Was this a lucky guess by Joseph Smith?

See:

Jeff Lindsay – Questions about the Book of Abraham, Part 3: Does It Agree with other Ancient Texts?

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