is attack on titan (shingeki no kyojin) episode 21 filler?

As the soldiers continue their retreat, Erwin and Hange discuss the way the Female Titan escaped. Erwin believes that it used the same tactic as the Colossal Titan, using omni-directional mobility gear to flee in the steam.

Hange argues that back then, Eren's gear was destroyed and he had no stamina left after his transformation, however, Erwin claims that using Eren as a basis for their inference was wrong, seeing that he is a novice. Meanwhile, Armin and Jean wonder if the person inside the Titan was captured.