He thought Black Swans (Cygnus atratus) did not even exist. Black Swans existed in abundance circa 100 AD - in Australia.
They're a species of Least Concern today.
His assumptions were incorrect because -he- and the scientific community at the time had rarely observed them in their fields of travel. But they existed - and in plentiful numbers.
I think Taleb has it correct. So did SECDEF Rumsfeld ("Unknown unknowns"). I can imagine what some people would do today if they saw a live Black Swan.
Tchaikovsky named his black swan Odile.