The 10 plagues of Revelation, similar to the 10 plagues of Egypt, are literal. Specifically Rev. 16:19 is literal according to the context. One cannot view the first half of a passage as literal and the second half as symbolic. It's either literal or symbolic.
18 And lightnings and voices and thunders occurred, and a great earthquake occurred such as had not occurred since men came to be on the earth, so extensive an earthquake, so great.
19 And the great city split into three parts, and the cities of the nations fell; and Babylon the Great was remembered in the sight of God, to give her the cup of the wine of the anger of his wrath.
20 Also, every island fled, and mountains were not found.
21 And a great hail with every stone about the weight of a talent descended out of heaven upon the men, and the men blasphemed God due to the plague of hail, because the plague of it was unusually great. (Rev. 16:18-21 NW)
P.S. Perhaps they got the idea from Isaiah. He informs us that divine hail "will sweep away the refuge of lies," it being mentioned alongside justice and righteousness (Is. 28:17 ESV). Nothing to do with hail-like judgment messages directed at the nations.