“Shortly” is a difficult concept in prophetic passages, as it can mean a short time, or that the end will come sooner than any expect it - even though that end might be over 2400 years as have passed since Nephi predicted that these things would come “shortly.” It may be also that this is the kind of prophecy with a millennial fulfilment, and perhaps some touchpoints of fulfillment along the way. The arrival of Christ in the New World was presaged by fire and vapor of smoke and the destruction of many of the wicked.
It may also be that the nature of prophecy is to compress time, and Nephi may be referring to the events at the time of Christ, and not see from his vantage point the expanse of time between the time of Christ and the beginning of the millenium. Perhaps for the prophets it is as it is with the Lord: “But they reside in the presence of God, on a globe like a sea of glass and fire, where all things for their glory are manifest, past, present, and future, and are continually before the Lord.” (D&C 130:7).