Because they know what Hashirama fought when they were infants? Tsunade is questioning if Hashirama fought it. Given that we have more knowledge on the fight than her and saw it for ourselves with no trace of Perfect Susanoo, we can throw out her words.This shows that Uchiha are the strongest and the only ones able to achive their power on their own through their genetics such as sharingan/MS/EMS > Rinnegan/Sharinnegan - and the only one who could stood up against Madara is Hashirama , senju were lucky they had someone like him. Everybody else are nothing infront of the Uchiha clan
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