If Oda writes every Seraphim losing purely because they turned their flame off unnecessarily then he is a terrible writer.
Don't introduce an overpowered ability if you're gonna write yourself in circles to manage it.
The only way Lunarian abilities make sense is if ACoC/top-tier AP is required to damage them with their flame-on. Or if a character is so strong that the Lunarian needs their flame-off otherwise they might not lose but they definitely can't win, e.g - why King would have his flame off vs Aramaki.
Oda restricting Luffy and Zoro's ACoC thus far is extremely telling. They're here using high-end attacks from their rooftop versions - that tells me that their current high-end attacks are overkill and Oda is trying to maintain tension. As well as not having Zoro brutally maim literal children.
We already knew Shi Shishi Sonson was ineffective vs flame-on Lunarians as King no-sold rooftop Zoro's. We already knew rooftop Zoro's AP was rooftop G4+ level as FDB was the first attack Kaido dodged. So really this chapter tells us nothing we didn't already know. It only comes as a shock as people either didn't want to admit how tough King is or couldn't correctly process it in their mind. We already knew flame-on King's defense was above Kaido's, so Luffy comparing a Lunarian to Kaido here is no surprise. The same Zoro that scarred Kaido thought there was no point in attacking a Lunarian with their flame-on. And Zoro remains the only person in the manga to ever scare a Lunarian in defense mode.