How was Joseph Smith so educated on Israelite law and justice in the case of Seantum’s confession?

The story of Seantum in the Book of Mormon is curious because his confession appears to be enough to convict him even though there were no witnesses. This goes against biblical traditions in which two witnesses are required. If Joseph Smith wrote the Book of Mormon, why would he include a case like this which would seem obviously untrue?

John Welch explains how in the Old Testament there are four episodes that support the idea that self-incriminating confessions could be used under certain circumstances in justifying punishment for unobserved criminal acts:

The rabbis explained that the four early biblical cases did not violate the two-witness rule, on several possible grounds: because they were confessions outside of court, because they came “after [the] trial and conviction [and were] made for the sole purpose of expiating the sin before God,” or because they were “exceptions to the general rule . . . [since they were] related to proceedings before kings or rulers” instead of before judges.
– John W. Welch – The Trial of Seantum

Welch continues:
Thus one can conclude with reasonable confidence that, in the biblical period, the two-witness rule could be overridden in the case of a self-incriminating confession, but not easily, and only if (1) the confession occurred outside the court or the will of God was evidenced in the detection of the offender, and (2) corroborating physical evidence was produced proving who committed the crime. Quite remarkably, Seantum’s self-incriminating confession was precisely such a case on all counts, and thus his execution would not have been legally problematic. His confession was spontaneous and occurred outside of court. The evidence of God’s will was supplied through Nephi’s prophecy. The tangible evidence was present in the blood found on Seantum’s cloak. The combination of these circumstances would have overridden the normal concerns in biblical jurisprudence about using self-incriminating confessions to obtain a conviction.
– John W. Welch – The Trial of Seantum

How would Joseph Smith have known this exception to the two-witness rule in the Bible? Where did he learn these intricacies of biblical law?

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