is attack on titan (shingeki no kyojin) episode 18 filler?

After the Female Titan leaves for Eren, Reiner asks Armin about his omni-directional mobility gear while tending his wounds. Armin replies that it seems to be working properly.

Reiner then notes that they have only one horse, with Jean attempting to call his own. He thinks to himself about what they will do, knowing that someone will have to be left behind.

Meanwhile, Armin seems to be spacing out, remembering his encounter with the Female Titan. Reiner then also says that they have to decide the one who will stay behind.