Sorry, I must have completely blanked and missed that statement. Regardless, it is compared to AGI, which is possibly one of the worst possible metrics to use.

Firstly, AGI is post-deductions, so people who play games with their taxes show up as paying more.

Secondarily, and more importantly, income is a trash metric anyways. Poor does not mean low income, it means low wealth. In your data somebody who inherits a ton of money but has 0 income counts as "bottom 50%". Rich people have a disproportionally lower income vs wealth.