It's only a fact in one's imagination, as concrete as a fact about properties of any other thing that can only be measured indirectly.
We have no way to directly measure the charge of one electron, so it's a fanciful claim, or at best an approximation, and not a concrete fact.
Claims about wires can be verified.
Are you suggesting the oil drop experiment was invalid?
It's only a fact in one's imagination, as concrete as a fact about properties of any other thing that can only be measured indirectly.
We have no way to directly measure the charge of one electron, so it's a fanciful claim, or at best an approximation, and not a concrete fact.
Claims about wires can be verified.