According to that article, riders experience about 5 gs at the bottom of the hill. So, a 100 lb rider would feel as if they were 500 lbs. I think the worry of lift would be during the initial descent, or more likely, at the top of the second hill. Thus, I am sure, the reason for the netting. I do not know how much the raft slows down do to hitting water at the bottom of the 1st hill. The article says the "raft slows a bit before a blast of water propels" it over the second hill. If significant enough, a sudden raft slowdown could result in an unsecured rider being "thrown" in front of the raft and either subject to the water blast or having the raft thrust over them. The description of "slows a bit" is hard to interpret. Flying out at the top of the second hill could still result in hitting the framework structure of the netting system. I don't think a gap would be necessary for a catastrophic result .
As a side note, that article says that the 14 yr old age restriction was lifted, but a height requirement was still in place.