He’s saying that IF the 1913 rates AND brackets, applied, but adjusting for inflation, that someone making 750 K would pay 2.5K in taxes. Not complicated.
OK, but I think the author could have illustrated his point better; and his point was the 16th amendment “. . . was a simple little thing, with rather large consequences for the republic.”
By my math using the tax computations worksheet someone in 2023 having a taxable income of $750,000 would pay over $209,000 of federal income taxes - - not $2,500. Set aside FICA taxes up to 15.3 percent (yes, I know I'm supposed to pretend the employer pays half.)
If the author wanted to reveal the “large consequences” of the 16th, he missed by quite a lot. Today we can thank the 16th for the upper rate of 37 percent.