The 3 billionaires mentioned were pre-1913. Morgan might have had a little bit of overlap.

The 90% tax rate was indeed in effect but very few people fell victim to it: http://almostclassical.blogspot.com/2011/03/90-tax-rate-myth...

Remember they all made their fortunes pre-1913 so those monies weren't subject to taxes when they earned them. They lived well past 1913, but by then they were giving most of it away. Post 1913 they paid income tax on the money they earned it was just being giving away.