You'd probably want to be more precise regarding "important for what" as well. And the philosophical angle: if the person hadn't existed, would someone else have slotted in to take their place? Is the impact measured in years/decades, because the overall historical forces were heading in a direction anyway?

Is it like trying to say "the most important bacterium in a petri dish"?

Without diminishing the impact that Musk has had, I'm fairly certain that Musk isn't the answer. And either way, the intent of saying that Musk is the most important person in history I'm fairly certain wasn't a very grounded decision. I'm sure it was more an expression of reverence and fealty.