He’s not the richest in history. Modern history, sure if you count excluding inflation. But when people like Mansa Musa existed Musk has quite a ways to go. Hell even Carnegie’s wealth eclipsed Musk if you’re talking purchasing power.
Musk has now surpassed Carnegie. You are right about Mansa Musa (for now), but the gap is closing. According to google today, Musk is at $334B, Carnegie peaked at a modest $309B, and Mansa Musa had about $400B, though there are quibbles about personal wealth vs. kingdom’s treasury. If you want to compare Musk and Musa, I think you could reasonably add the GDP of the US to Musk’s fortune, since he is one of the puppeteers behind Trump. All figures adjusted for inflation, by the way.