How come Joseph Smith knew so much about Exodus 21:13 and Biblical law to make the story of Nephi killing Laban legally justifiable?

The story of Nephi killing Laban is a major stumbling block to many readers, however John Welch notes that:

Nephi may have broken the American law of Joseph Smith’s day, but it appears that he committed an excusable homicide under the public law of his own day… if an action had been brought against Nephi, early biblical law appears to have recognized two types of killings-excusable and inexcusable-and the slaying of Laban arguably falls quite specifically into the excusable category.
– John W. Welch – Legal Perspectives on the Slaying of Laban

This is based on Exodus 21:12-13 which says:

He that smiteth a man, so that he die, shall be surely put to death.
And if a man lie not in wait, but God deliver him into his hand; then I will appoint thee a place whither he shall flee.

From this it seems there are excusable killings if two criteria are met:

the slayer must not have been lying in wait
God must deliver the victim into the slayer’s hand.

1 Nephi 4:6,11 address both these points quite clearly.

And I was led by the Spirit, not knowing beforehand the things which I should do
And the Spirit said unto me again: Behold the Lord hath delivered him into thy hands.

How would Joseph know to include both these points, including the exact word “deliver”?

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