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But he doesn't know anything about Gedo Mazo. So the best he can come up with is "its a big statue so it can probably smash rocks" or something. Coming up with the conclusion that it can rip souls out of nowhere is a little far-fetched...
Same with the Bijuus. I don't think any of us thought that the Bijuus could casually bust mountains with TBBs and create
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when combining their power. And unless Kabuto's researched every Jinchuuriki, he won't know that Roshi has lava, Utakata has explosive bubbles, Yagura can reflect any attack, etc. He'll just think that they're vessels for the Bijuus. He won't know that Yagura can reflect Hashirama's attacks, etc.
And it still stands that he knows nothing about Izanagi (maybe he does now after being put in Izanami), Tobi's Genjutsu (killed Konan with it + controlled a perfect Jinchuuriki), or his S/T techs (Tobi hasn't used them in front of Kabuto).
And
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Kabuto says that he's invincible though he knows all about "the fusion of Madara and Hashirama" in his arsenal, so his statement about Hashirama loses more credibility. And
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Kabuto said that "there are no Shinobi like him anymore"....the same thing Kishi said about Minato. See
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for the same translation regarding Hashirama.