The only known trebuchet casualty in history

Trebuchets crushed their victims so hard that they were erased from history.

Trebuchet were used primarily to hammer the walls of the castle to break them down. Not really possible to aim at a person; the soldiers clambering over the broken walls will do that.

People didn't really bother recording cause of death for every casualty on medieval battlefields

the key word is "known"

They typically weren't trying to hit people with them.