To me it's clear that Oda retconned Luffy's DF to a mythical Zooan, but I'm not sure when. It might be earlier than expected.
It's interesting noticing that in every one of Luffy's major battles Oda is starting to hint that Luffy's DF is more than just rubber.
In Dressrosa
In WCI
In Wano
Kaido's comment was the most perceptive, outright declaring that simple rubber is not enough to do that. Although this is clearly Oda setting up the upcoming transformation.
It's interesting noticing that in every one of Luffy's major battles Oda is starting to hint that Luffy's DF is more than just rubber.
In Dressrosa
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In WCI
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In Wano
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Kaido's comment was the most perceptive, outright declaring that simple rubber is not enough to do that. Although this is clearly Oda setting up the upcoming transformation.