Oh also Marco can turn himself into fire / flames of the phoenix. So it's not totally unprecedented for a mythical zoan to have some paramecium type properties. Although that's an active ability not a passive one like Luffy.
There's also a lot of foreshadowing for this from Skypeia. Luffy's silhouette while he's dancing around the fire matches the Nika silhouette shown in Wano and he's dancing the beat of the drums (which have the same 'sound marks' as the drums of liberation.
Plus just in general, we've never really seen another paramecium fruit that allows the user to have fundamentally different forms, so it really does track for him to actually have a zoan.
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The Artist Formerly Known as Hannyabal
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