First, we know the result of what transpired in Danzo's clash with Shisui though. Shisui lost an eye..Saying we don't know all that happened is probably better.you dont know exactly what transpired during the danzo and shisui clash, the exact details which is necessary and conditions so you making a conclusion at bold is rather premature.
well considering they would be up against all the clans in konoha and they are only one without any ally of course danzo would confident they would be wiped out, numbers wise alone, odds are in the favour of the village. one clan against what? multiple? not sure how that shows they are not strong, it depicts the opposite if anything.
not to mention their reputation for being the most feared still remained during the era of the coup and even after the massacre, kakashi still referenced them as the strongest in the leaf. this shows that uchiha were still the no 1 clan during that time. so portrayal is also in their favour
Second..@bold make no mistakes..i never came to any conclusion myself...Look at the scan again
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Middle portion.. scan on the right..second word bubble. Danzo clearly says "....whole clan destroyed by a shinobi of the leaf..." I never made such a claim..the manga did itself and i only posted it here to reply to OP's question.