In many ways, yes, this description of war seems realistic. The speaker has described how incredibly exhausted he and the other troops are. Though they are young men, they are "bent double, like old beggars" and "coughing like hags." The soldiers are so tired that they seem to march "asleep"; further, many have no boots and must "trudge" through the "sludge" with bloodied feet. Everyone, despite their youth, seems to have gone "lame" and "blind" and are so exhausted that they seem "drunk" and "deaf," as though even sounds barely register in their overwhelmed brains.
However, the mood changes quickly with the second group of lines, after "gas-shells" drop near them. Someone realizes what's happening and calls out to the others; the capitalization of "GAS!" helps to emphasize the panic they must feel as they "fumbl[e]" to get their gas masks on before breathing in the putrid and deadly gas released from the shell. One soldier, though, does not get his mask on quickly enough and seems to be "flound'ring like a man in fire or lime." The speaker describes this man as appearing to "drown" in a "green sea." He describes the "misty panes" of his gas mask, which prevent him from seeing very clearly, as well as the "thick green light" apparently created by the gaseous fog. Although the speaker's descriptions are somewhat figurative, they create very clear images of panic and confusion, of tragedy
and helplessness. Once his fellow has inhaled the gas, there is nothing anyone can do to stop him from "guttering, choking, drowning" in the blood that fills his "froth-corrupted lungs." Such a description is meant to evoke reality, as the speaker has a clear agenda to present war truthfully rather than in some glorified way that feels innately dishonest.