The author says that J. P. Morgan's estate was worth $1.5bn in 2014 dollars, not in 1913 dollars.

Apologies: I stand corrected.

Speculation only, but I'd expect the relative size and scope of the financial industry today vs 1913 to explain that difference.

As in it's easier to earn $1.5bn within 2013's financial industry than it would have been (inflation adjusted) in 1913.

Yeah, this is more or less right. Inflation adjustments measure how much you have to spend to consume a given amount -- so, basically, if a loaf of bread costs $0.02 in 1913 and $2.00 in 2014, then inflation is 100x. With lots of complexity in there as well, of course.

Notably, what inflation isn't is something like "percentage of total size of the economy." So if the economy expands (as it has), then the amount of money available to be made in something that scales to the size of the economy (like the financial markets) increases well beyond the pace of inflation.

In 1913, the US population was about 1/3rd what it is now. The US now plays a much more significant role in the world economy, and of course technological advancement has increased the size of the economy as well. Inflation doesn't account for any of that per se.

Note that in 1913 U.S. economy as a whole was probably 20 times smaller, even after adjusting for inflation. So, JP Morgan had a relatively big chunk of money as a percentage of real gdp

Moreover, capital was far scarcer then than now. If you needed to raise money for an enterprise, there were fewer places to look for it. Consequently, Ol' Pierpont had a great deal more politial power than someone with the same amount of cash might today.

First sentence: "a fortune of about $1.5 billion in today’s dollars."