Mormon was a military man, and as such understood much of the order of battle that he neglects to write down. It is up to us to fill in the blanks he leaves us. Zerahemnah does not come up to siege without any provisions. He does bring them, but they are necessarily limited. They are intended to be replenished, and one of the sources of replenishment is apparently intended to be the closer areas of “wilderness” to the Nephite stronghold. This would have been supplemental supplies. Mormon specifically notes that they hunted for meat. The grains would have been with them already.
From the earlier verses we learn that the difficulty of raising crops was the susceptibility to Nephite raiding parties. In these verses we see Nephite raiding parties. While there is no mention of attempts to raise food from the land, it is almost a requirement. A siege requires a longer time in the field, and thus the pressures on a supply line are even greater than those on Giddianhi’s supplies. The solution would have been to raise crops, and the Nephite solution was to harry the Gadianton farmers to make that very difficult if not impossible.
The Nephite position had to have been sufficiently localized that there was no easy access to the north, else a siege would have been impossible. Nevertheless, the area also appears to have been fairly large, as the Nephites were able to exit on sallies without being immediately stopped by the Gadianton army. This this is not the typical siege of a walled city, but of a larger defended area. The intent of the siege was to create supply problems inside the area. The irony was that the inside was better provisioned than the outside. In addition to the provisions they brought into this defended land, it is also quite probable that there were protected fields in which crops could be grown. Of course it goes without saying that there had to have been an abundant supply of water available.