The most interesting part of this verse is the “wherefore.” Nephi is linking Jesus’ actions in baptism to the Holy Ghost descending. The particular actions that appear to be linked are the reasons for his baptism, not the baptism per se. Thus the Holy Ghost descends because of the reasons that Jesus submits to the baptism. Of course in this he will also serve as an example to men, that they may also attain to the Holy Ghost following their own baptisms.
While the association of the Holy Spirit and baptism might appear to be a very Christian concept, the Holy Spirit also appears to be invoked for baptisms at Qumran:
Eisenman and Wise discuss a fragment of a text on baptism at Qumran and note:
“On the heels of this text, we come upon a series of fragments relating to baptism. By baptism, of course, the reader should realize that the proponents of this literature did not necessarily mean anything different from traditional Jewish ritual immersion. The terminologies are synonymous, though the emphasis on baptismal procedures at Qumran is extraordinary. This can be seen not only in texts such as the one represented by these fragments and the well-known Community Rule, iii, 1-4, which in describing baptism makes reference to ‘the Holy Spirit’, but also the sheer number of ritual immersion facilities at the actual ruins of Qumran - if these can he safely associated with the movement responsible for this literature.” (Eisenman, Robert, and Michael Wise. The Dead Sea Scrolls Uncovered. New York: Barnes and Noble. 1994, p. 230.)
While the community of the scrolls is certainly later than Nephi, they are yet prior to the Christian usage of baptism and the gift of the Holy Ghost. This linkage between baptism and the spirit is demonstrably pre-Christian, and while it cannot be firmly dated to anything Nephi would have taken with him from the Old World, it is not a concept that would be precluded prior to Christ’s birth - particularly since Nephi is declaring these things as part of his own vision.