I think it's not that Ivankov is Sanji's worst nightmare, it's more like he's remembering this time on okama island and Killingham having no clue what's going on gets the strongest okama as a "nightmare".
I still believe Sanji's worst fear is becoming just like his German bros (emotionless and cruel), but in a way, he's already fighting his evil self as Killingham is what Sanji could be should he take the #3 spot in Germa siblings team.
While I am as annoyed at the unfortunate break timing as everyone here, I actually am glad that Sanji got a full chapter to himself. We got one for Zoro (and I actually loved that fight, it was hilarious), why not get one for Sanji?
This way, the next chapter (1197) can be the chapter where BOTH Sanji and Zoro defeat their opponents with the power of CoC, we hopefully get a glimpse of what Lili left in the box (her diary maybe?), and we will get back to hybrid Loki and Imu fight.