Hey, thanks for the rep.
First off it is only a guess - I'm sure there are other potential solutions.
That said the prediction is based on more than Kaguya not being fooled by clones but there is that as well.
First, BZ comments that a clone would not be able to survive travel through the portal into this realm:
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That should rule out clones from being involved in this trick altogether.
Second is the comment from Naruto that the only things that work against Kaguya are diversions and taijutsu:
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That makes me think that this is an elaborate fake out dreamt up on the fly.
Third, if Naruto is not a clone, he is still not able to sustain such a blow while maintaining so many clones - at least that's BZ's reckoning:
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Of all the characters that are left, only one is non essential:
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...and only one can take a blow like that, recover and maintain a clone (a "Sakura" is still with Obito but a clone would be enough to deliver chakra which is what she's there for):
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If Kaguya had had her Byakugan active then she might have been able to read the chakra flow and determine that it wasn't Naruto but there aren't any veins bulging and Kaguya is saving on chakra for something else in the works:
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Generally speaking if Kaguya just had Byakugan (like Neji) then I figure she might be taken in by clones alone but her Sharingan / Rinnegan should be able to do everything Madara was able to do and he could tell the difference between clones and real people:
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Taking everything into account I think that that just leaves a Transformation Jutsu and Sakura. I've read a couple of the theories out there but none of them seem all that convincing. Given the setup I don't think clones are being employed and I don't expect a character to die quite yet. At least that's my take.