[sworn by all that had been forbidden them by our Lord…]: The reference to this statement is the oaths sworn in verse 10 above. Here Mormon explicitly ties these oaths to their context in the Sermon on the Mount. As indicated in the commentary following 3 Nephi 12:34 which is the source for Mormon’s current statement, the particular oaths occur in the context of second temple Jerusalem, and may not have any relevance to a Mesomamerican audience. However, Mormon indicates that there was relevance in the denial of at least certain types of oaths to the Nephites.
One indication that this is a weak point in the textual transmission is that the current reference to 3 Nephi is imprecise, and makes an error in making an explanation into a prohibited oath.
In the Matthean version we have:
Matthew 5:34
34 But I say unto you, Swear not at all; neither by heaven; for it is God’s throne:
The version in 3 Nephi is virtually identical, and specifically identical in the section dealing with the prohibited oath:
3 Nephi 12:34
34 But verily, verily, I say unto you, swear not at all; neither by heaven, for it is God’s throne;
Both of these verses agree that the prohibited oath is swearing by heaven, and the reason that one should not swear by heaven is that it is God’s throne, and therefore sacred. Contrast that clear separation between oath and reason to the specifics given for the Nephite oaths: