Here Joseph Smith added as to make the text reflect a less literal interpretation for the “lake of fire and brimstone”. This emendation makes the passage parallel to two other passages that use as to specifically refer to the metaphorical nature of the lake of fire and brimstone:
We do not have the original manuscript for 2 Nephi 9:16, so it is possible that as was accidentally dropped in the early transmission of the text. Obviously, the phraseology is virtually the same for all three passages.
In support of this emendation, there are a number of cases where the scribes dropped as in the manuscripts. The following examples show the scribe (Oliver Cowdery, except in one case) initially omitting an as as he copied from 𝓞 into 𝓟:
It should be noted that, like 2 Nephi 9:16, one of the above passages (Alma 37:44) involves omitting an as after a form of the verb be.
On the other hand, there are several examples in the text that directly equate “the lake of fire and brimstone” with “endless torment”:
David Calabro points out (personal communication) that in each of these examples the simile comes first (“the lake of fire and brimstone is endless torment”):
The use of as for any of these would seem odd (“the lake of fire and brimstone is as endless torment”). On the other hand, in the examples that have as, the simile comes second (“their torment is as a lake of fire and brimstone”):
The very close parallelism of these three examples strongly argues for Joseph Smith’s emendation in the first one and further suggests that the as may have been accidentally omitted in the early transmission of the text.
Nonetheless, there is an important connection between 2 Nephi 9:16 and the four examples which state that “the lake of fire and brimstone is endless torment”: all five are found on the small plates. In fact, for four of them, the writer is Jacob (and three of them are in this same chapter, 2 Nephi 9); Nephi is the author of the other one (in 2 Nephi 28:23). And for every one of these cases, Jacob and Nephi specifically treat “the lake of fire and brimstone” as a real entity. There is no hint that they might think this lake is metaphorical. On the other hand, the two examples that use as are found in Mormon’s abridgment of the large plates, with king Benjamin speaking in Mosiah 3:27 and Alma in Alma 12:17. In other words, Jacob himself prefers a literal interpretation for “the lake of fire and brimstone”. Thus the earliest reading in 2 Nephi 9:16 is perfectly acceptable, given Jacob’s language elsewhere. For this reason, the critical text will restore the earliest reading in 2 Nephi 9:16 without the as, even though the possibility remains that an original as may have been lost during the early transmission of the text.
Summary: Maintain in 2 Nephi 9:16 the earliest text, which lacks the as after is (“and their torment is a lake of fire and brimstone”); Joseph Smith’s addition of the word as parallels the language found in Mormon’s abridgment of the large plates (Mosiah 3:27 and Alma 12:17), but in the small plates Jacob (as well as Nephi) refers to the lake of fire and brimstone as actually existing.