is attack on titan (shingeki no kyojin) episode 30 filler?

Some years ago, the cadets of the 104th Cadet Corps complete a snowstorm training exercise, arriving at a base at the foot of a mountain. After some brief discussion among the cadets, Reiner announces that Christa is missing.

Mikasa says that Christa had chosen to stick with Daz, who was performing poorly, but Reiner says that Daz as well as Ymir have not returned. Marco suggests that she may be with Thomas' squad, but Reiner points out that his squad just completed their course with Ymir, Daz, and Christa nowhere in sight.