He offpaneled Luffy's ACoA training as well. That isn't the issue.
The point is Enma had -
1. Narrative Significance : To cut Kaido. Just like Luffy's - to hurt Kaido.
2. Wano centric Plotline: Had Zoro lose Shusui and search for it leading to the Hiyori reveal/plotline and ending with him receiving Enma. Also gave more lore about Ryuma, Shusui and Wano. Just like Luffy had to train in the prisons and ended up freeing the Samurai of their physical and mental chains over there. Also brought in Hyogoro, Kawamatsu, Big Mom, Queen and tied together several plotlines.
3. A training arc, just like how Luffy trained his ACoA.
4. Foreshadowing for future powerups: Zoro being told that he can make Enma a black blade in the future, hinting something that 100% is going to happen by the end of this arc and probably is his powerup against King. Exactly like Luffers learning how to make his haki flow, leading to him using AdvCoC by making his CoC flow.
Guess I was wrong for expecting something similar for Sanji.