Is Musk the richest American ever now? No, except as a consumer
The richest American was Rockefeller, who 100 years ago was worth around $1b ($700m-1.2b estimated).
$1b in 1900 USD equates to $25b in 2026 USD, which is fraction of Musk's worth, but this is only in purchasing power as a consumer.
In basically all metrics common to 1900 and 2026, Musk could buy more.
Musk can buy 140b hours of work at minimum wage ($7.25), whereas Rockefeller could buy only 4.5b hours at the time. Musk could buy 7500 tons of gold versus Rockefeller's 1500 (5x). 230m acres of US farmland versus 50m (5x). 70m metric tons of copper versus 3m (25x). 1600 barrel board feet of lumber versus 85b.
Rockefeller still was richer, though, because of what the money made him and empowered him to do (less laws that limit use of money, labor laws, antitrust laws, etc, less accountability to courts for various reasons). In the early 1900s, a man could buy and own, completely, almost anything. (For example, in 1901 Carnegie sold Carnegie Steel for $480m, and with that single transaction JP Morgan bought a functional monopoly on the metal that built basically everything in the US, and today $1t perhaps can't buy any monopoly. A few hundred million dollars was also enough to bail out a country 100 years ago (JP Morgan when US ran out of gold). The leverage involved in this type of ability is obvious.
In 2026 $1t just makes you a massive player, among many players, so many that the biggest ones don't really matter economically (although in Musk's case, obviously, we all value him for things besides economics).
The value of money is what holding it makes you and what spending it can get you. It could be argued (seriously or not I'm not sure) that some billionaire in some less restraining country in Africa is richer than Musk in terms of what his money means. It's also worth noting, although maybe it's obvious, that in terms of technological wealth, poor people today are richer than Rockefeller.