You contradict yourself.

Just because we know the Romans used lead, doesn't mean they were the only ones or the only possible cause (both unknown unknowns). There's no contradiction, there's an unshared set of assumptions.

Yeah i think Julius Ceasar global leadening was a conspriacy by China /s

How does that random thought happen?

Well, your second sentence is true. But since the levels seem to rise and fall with conditions in Rome - a known polluter with known health problems from lead - it is a very likely source of those levels.

Scientists create hypotheses on the basis of information available to them; if more information arrives, the hypotheses are (eventually) adjusted accordingly. They're not meant to be taken as absolute assertions.