That source says that Rockefeller's net worth was $30 billion in current (2014) dollars.

The $340 billion figure is from a semi-nonsensical calculation that is popular because it yields eye-popping large numbers. They figured Rockefeller's assets equaled 1.5% of the US GDP at the time. And 1.5% of the current GDP would be about $340 billion dollars. But assets as a fraction of GDP is not very meaningful. It's like saying if I moved to a country with a small GDP such as Samoa I'd be richer than Bill Gates.

In effect, the $340B Rockefeller number is big because the US economy has grown. The worst part is many websites often confuse the scaled-GDP numbers with constant-dollar numbers, resulting in much confusion.